Aptamer-functionalized neural recording electrodes for the direct measurement of cocaine in vivo

Pursuing precision in medicine and nutrition: the rise of electrochemical biosensing at the molecular level

In the era that we seek personalization in material things, it is becoming increasingly clear that the individualized management of medicine and nutrition plays a key role in life expectancy and quality of life, allowing participation to some extent in our welfare and the use of societal resources in a rationale and equitable way. The implementation of precision medicine and nutrition are highly complex challenges which depend on the development of new technologies able to meet important requirements in terms of cost, simplicity, and versatility, and to determine both individually and simultaneously, almost in real time and with the required sensitivity and reliability, molecular markers of different omics levels in biofluids extracted, secreted (either naturally or stimulated), or circulating in the body. Relying on representative and pioneering examples, this review article critically discusses recent advances driving the position of electrochemical bioplatforms as one of the winning horses for the implementation of suitable tools for advanced diagnostics, therapy, and precision nutrition. In addition to a critical overview of the state of the art, including groundbreaking applications and challenges ahead, the article concludes with a personal vision of the imminent roadmap.

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

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

Survey of oligoethylene glycol-based self-assembled monolayers on electrochemical aptamer-based sensor in biological fluids

The ability to monitor levels of endogenous markers and clearance profiles of drugs and their metabolites can improve the quality of biomedical research and precision with which therapies are individualized. Towards this end, electrochemical aptamer-based (EAB) sensors have been developed that support the real-time monitoring of specific analytes in vivo with clinically relevant specificity and sensitivity. A challenge associated with the in vivo deployment of EAB sensors, however, is how to manage the signal drift which, although correctable, ultimately leads to unacceptably low signal-to-noise ratios, limiting the measurement duration. Motivated by the correction of signal drift, in this paper, we have explored the use of oligoethylene glycol (OEG), a widely employed antifouling coating, to reduce the signal drift in EAB sensors. Counter to expectations, however, when challenged in 37 °C whole blood in vitro, EAB sensors employing OEG-modified self-assembled monolayers exhibit both greater drift and reduced signal gain, compared with those employ a simple, hydroxyl-terminated monolayer. On the other hand, when EAB sensor was prepared with a mix monolayer using MCH and lipoamido OEG 2 alcohol, reduced signal noise was observed compared to the same sensor prepared with MCH presumably due to improved SAM construction. These results suggest broader exploration of antifouling materials will be required to improve the signal drift of EAB sensors.

Continuous Molecular Monitoring in the Body via Nucleic Acid-based Electrochemical Sensors: The Need for Statistically-powered Validation

Nucleic acid-based electrochemical (NBE) sensors offer real-time and reagent-free sensing capabilities that overcome limitations of target-specific reactivity via affinity-based molecular detection. By leveraging affinity probes, NBE sensors become modular and versatile, allowing the monitoring of a variety of molecular targets by simply swapping the recognition probe without the need to change their sensor architecture. However, NBE sensors have not been rigorously validated in vivo in terms of analytical performance and clinical agreement relative to benchmark methods. In this article, we highlight reports from the past three years that evaluate NBE sensors performance in vivo. We hope this discussion will inspire future translational efforts with statistically robust experimental design, thus enabling real-world clinical applications and commercial development of NBE sensors.

High-precision monitoring of and feedback control over drug concentrations in the brains of freely moving rats

Knowledge of drug concentrations in the brains of behaving subjects remains constrained on a number of dimensions, including poor temporal resolution and lack of real-time data. Here, however, we demonstrate the ability of electrochemical aptamer-based sensors to support seconds-resolved, real-time measurements of drug concentrations in the brains of freely moving rats. Specifically, using such sensors, we achieve <4 μM limits of detection and 10-s resolution in the measurement of procaine in the brains of freely moving rats, permitting the determination of the pharmacokinetics and concentration-behavior relations of the drug with high precision for individual subjects. In parallel, we have used closed-loop feedback-controlled drug delivery to hold intracranial procaine levels constant (±10%) for >1.5 hours. These results demonstrate the utility of such sensors in (i) the determination of the site-specific, seconds-resolved neuropharmacokinetics, (ii) enabling the study of individual subject neuropharmacokinetics and concentration-response relations, and (iii) performing high-precision control over intracranial drug levels.

Efforts toward the continuous monitoring of molecular markers of performance

OBJECTIVES

Technologies supporting the continuous, real-time measurement of blood oxygen saturation and plasma glucose levels have improved our ability to monitor performance status. Our ability to monitor other molecular markers of performance, however, including the hormones known to indicate overtraining and general health, has lagged. That is, although a number of other molecular markers of performance status have been identified, we have struggled to develop viable technologies supporting their real-time monitoring in the body. Here we review biosensor approaches that may support such measurements, as well as the molecules potentially of greatest interest to monitor.

DESIGN

Narrative literature review.

METHOD

Literature review.

RESULTS

Significant effort has been made to harness the specificity, affinity, and generalizability of biomolecular recognition in a platform technology supporting continuous in vivo molecular measurements. Most biosensor approaches, however, are either not generalizable to most targets, or fail when challenged in the complex environments found in vivo. Electrochemical aptamer-based sensors, in contrast, are the first technology to simultaneously achieve both of these critical attributes. In an effort to illustrate the potential of this platform technology, we both critically review the literature describing it and briefly survey some of the molecular performance markers we believe will prove advantageous to monitor using it.

CONCLUSIONS

Electrochemical aptamer-based sensors may be the first truly generalizable technology for monitoring specific molecules in situ in the body and how adaptation of the platform to subcutaneous microneedles will enable the real-time monitoring of performance markers via a wearable, minimally invasive device.

Real-Time, Seconds-Resolved Measurements of Plasma Methotrexate In Situ in the Living Body

Dose-limiting toxicity and significant patient-to-patient pharmacokinetic variability often render it difficult to achieve the safe and effective dosing of drugs. This is further compounded by the slow, cumbersome nature of the analytical methods used to monitor patient-specific pharmacokinetics, which inevitably rely on blood draws followed by post-facto laboratory analysis. Motivated by the pressing need for improved "therapeutic drug monitoring", we are developing electrochemical aptamer-based (EAB) sensors, a minimally invasive biosensor architecture that can provide real-time, seconds-resolved measurements of drug levels in situ in the living body. A key advantage of EAB sensors is that they are generalizable to the detection of a wide range of therapeutic agents because they are independent of the chemical or enzymatic reactivity of their targets. Three of the four therapeutic drug classes that have, to date, been shown measurable using in vivo EAB sensors, however, bind to nucleic acids as part of their mode of action, leaving open questions regarding the extent to which the approach can be generalized to therapeutics that do not. Here, we demonstrate real-time, in vivo measurements of plasma methotrexate, an antimetabolite (a mode of action not reliant on DNA binding) chemotherapeutic, following human-relevant dosing in a live rat animal model. By providing hundreds of drug concentration values, the resulting seconds-resolved measurements succeed in defining key pharmacokinetic parameters, including the drug's elimination rate, peak plasma concentration, and exposure (area under the curve), with unprecedented 5 to 10% precision. With this level of precision, we easily identify significant (>2-fold) differences in drug exposure occurring between even healthy rats given the same mass-adjusted methotrexate dose. By providing a real-time, seconds-resolved window into methotrexate pharmacokinetics, such measurements can be used to precisely "individualize" the dosing of this significantly toxic yet vitally important chemotherapeutic.

Zwitterionic Polymer Coated and Aptamer Functionalized Flexible Micro-Electrode Arrays for In Vivo Cocaine Sensing and Electrophysiology

The number of people aged 12 years and older using illicit drugs reached 59.3 million in 2020, among which 5.2 million are cocaine users based on the national data. In order to fully understand cocaine addiction and develop effective therapies, a tool is needed to reliably measure real-time cocaine concentration and neural activity in different regions of the brain with high spatial and temporal resolution. Integrated biochemical sensing devices based upon flexible microelectrode arrays (MEA) have emerged as a powerful tool for such purposes; however, MEAs suffer from undesired biofouling and inflammatory reactions, while those with immobilized biologic sensing elements experience additional failures due to biomolecule degradation. Aptasensors are powerful tools for building highly selective sensors for analytes that have been difficult to detect. In this work, DNA aptamer-based electrochemical cocaine sensors were integrated on flexible MEAs and protected with an antifouling zwitterionic poly (sulfobetaine methacrylate) (PSB) coating, in order to prevent sensors from biofouling and degradation by the host tissue. In vitro experiments showed that without the PSB coating, both adsorption of plasma protein albumin and exposure to DNase-1 enzyme have detrimental effects on sensor performance, decreasing signal amplitude and the sensitivity of the sensors. Albumin adsorption caused a 44.4% sensitivity loss, and DNase-1 exposure for 24 hr resulted in a 57.2% sensitivity reduction. The PSB coating successfully protected sensors from albumin fouling and DNase-1 enzyme digestion. In vivo tests showed that the PSB coated MEA aptasensors can detect repeated cocaine infusions in the brain for 3 hrs after implantation without sensitivity degradation. Additionally, the same MEAs can record electrophysiological signals at different tissue depths simultaneously. This novel flexible MEA with integrated cocaine sensors can serve as a valuable tool for understanding the mechanisms of cocaine addiction, while the PSB coating technology can be generalized to improve all implantable devices suffering from biofouling and inflammatory host responses.

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.

Recent progress and perspectives of continuous in vivo testing device

Devices for continuous in-vivo testing (CIVT) can detect target substances in real time, thus providing a valuable window into a patient's condition, their response to therapeutics, metabolic activities, and neurotransmitter transmission in the brain. Therefore, CIVT devices have received increased attention because they are expected to greatly assist disease diagnosis and treatment and research on human pathogenesis. However, CIVT has been achieved for only a few markers, and it remains challenging to detect many key markers. Therefore, it is important to summarize the key technologies and methodologies of CIVT, and to examine the direction of future development of CIVT. We review recent progress in the development of CIVT devices, with consideration of the structure of these devices, principles governing continuous detection, and nanomaterials used for electrode modification. This detailed and comprehensive review of CIVT devices serves three purposes: (1) to summarize the advantages and disadvantages of existing devices, (2) to provide a reference for development of CIVT equipment to detect additional important markers, and (3) to discuss future prospects with emphasis on problems that must be overcome for further development of CIVT equipment. This review aims to promote progress in research on CIVT devices and contribute to future innovation in personalized medical treatments.

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A detailed and comprehensive review of continuous in vivo testing device.

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The nanomaterials, delicate structures and detection principles of the works are discussed.

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The achievements and shortcomings of the existing devices are summarized.

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The problems that should be solved in the further development of the devices and the future prospects are put forward.

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.

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.

Semi-Implantable Bioelectronics

Developing techniques to effectively and real-time monitor and regulate the interior environment of biological objects is significantly important for many biomedical engineering and scientific applications, including drug delivery, electrophysiological recording and regulation of intracellular activities. Semi-implantable bioelectronics is currently a hot spot in biomedical engineering research area, because it not only meets the increasing technical demands for precise detection or regulation of biological activities, but also provides a desirable platform for externally incorporating complex functionalities and electronic integration. Although there is less definition and summary to distinguish it from the well-reviewed non-invasive bioelectronics and fully implantable bioelectronics, semi-implantable bioelectronics have emerged as highly unique technology to boost the development of biochips and smart wearable device. Here, we reviewed the recent progress in this field and raised the concept of "Semi-implantable bioelectronics", summarizing the principle and strategies of semi-implantable device for cell applications and in vivo applications, discussing the typical methodologies to access to intracellular environment or in vivo environment, biosafety aspects and typical applications. This review is meaningful for understanding in-depth the design principles, materials fabrication techniques, device integration processes, cell/tissue penetration methodologies, biosafety aspects, and applications strategies that are essential to the development of future minimally invasive bioelectronics.

Carbon-Based Electrochemical Sensors for In Vivo and In Vitro Neurotransmitter Detection

As essential neurological chemical messengers, neurotransmitters play an integral role in the maintenance of normal mammalian physiology. Aberrant neurotransmitter activity is associated with a range of neurological conditions including Parkinson's disease, Alzheimer's disease, and Huntington's disease. Many studies to date have tested different approaches to detecting neurotransmitters, yet the detection of these materials within the brain, due to the complex environment of the brain and the rapid metabolism of neurotransmitters, remains challenging and an area of active research. There is a clear need for the development of novel neurotransmitter sensing technologies capable of rapidly and sensitively monitoring specific analytes within the brain without adversely impacting the local microenvironment in which they are implanted. Owing to their excellent sensitivity, portability, ease-of-use, amenability to microprocessing, and low cost, electrochemical sensors methods have been widely studied in the context of neurotransmitter monitoring. The present review, thus, surveys current progress in this research field, discussing developed electrochemical neurotransmitter sensors capable of detecting dopamine (DA), serotonin (5-HT), acetylcholine (Ach), glutamate (Glu), nitric oxide (NO), adenosine (ADO), and so on. Of these technologies, those based on carbon nanostructures-modified electrodes including carbon nanotubes (CNTs), graphene (GR), gaphdiyne (GDY), carbon nanofibers (CNFs), and derivatives thereof hold particular promise owing to their excellent biocompatibility and electrocatalytic performance. The continued development of these and related technologies is, thus, likely to lead to major advances in the clinical diagnosis of neurological diseases and the detection of novel biomarkers thereof.

Nanoparticle and Biomolecule Surface Modification Synergistically Increases Neural Electrode Recording Yield and Minimizes Inflammatory Host Response

Due to their ability to interface with neural tissues, neural electrodes are the key tool used for neurophysiological studies, electrochemical detection, brain computer interfacing, and countless neuromodulation therapies and diagnostic procedures. However, the long-term applications of neural electrodes are limited by the inflammatory host tissue response, decreasing detectable electrical signals, and insulating the device from the native environment. Surface modification methods are proposed to limit these detrimental responses but each has their own limitations. Here, a combinatorial approach is presented toward creating a stable interface between the electrode and host tissues. First, a thiolated nanoparticle (TNP) coating is utilized to increase the surface area and roughness. Next, the neural adhesion molecule L1 is immobilized to the nanoparticle modified substrate. In vitro, the combined nanotopographical and bioactive modifications (TNP+L1) elevate the bioactivity of L1, which is maintained for 28 d. In vivo, TNP+L1 modification improves the recording performance of the neural electrode arrays compared to TNP or L1 modification alone. Postmortem histology reveals greater neural cell density around the TNP+L1 coating while eliminating any inflammatory microglial encapsulation after 4 weeks. These results demonstrate that nanotopographical and bioactive modifications synergistically produce a seamless neural tissue interface for chronic neural implants.

Recent Advances in In Vivo Neurochemical Monitoring

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

Narrower Nanoribbon Biosensors Fabricated by Chemical Lift-off Lithography Show Higher Sensitivity

Wafer-scale nanoribbon field-effect transistor (FET) biosensors fabricated by straightforward top-down processes are demonstrated as sensing platforms with high sensitivity to a broad range of biological targets. Nanoribbons with 350 nm widths (700 nm pitch) were patterned by chemical lift-off lithography using high-throughput, low-cost commercial digital versatile disks (DVDs) as masters. Lift-off lithography was also used to pattern ribbons with 2 μm or 20 μm widths (4 or 40 μm pitches, respectively) using masters fabricated by photolithography. For all widths, highly aligned, quasi-one-dimensional (1D) ribbon arrays were produced over centimeter length scales by sputtering to deposit 20 nm thin-film In₂O₃ as the semiconductor. Compared to 20 μm wide microribbons, FET sensors with 350 nm wide nanoribbons showed higher sensitivity to pH over a broad range (pH 5 to 10). Nanoribbon FETs functionalized with a serotonin-specific aptamer demonstrated larger responses to equimolar serotonin in high ionic strength buffer than those of microribbon FETs. Field-effect transistors with 350 nm wide nanoribbons functionalized with single-stranded DNA showed greater sensitivity to detecting complementary DNA hybridization vs 20 μm microribbon FETs. In all, we illustrate facile fabrication and use of large-area, uniform In₂O₃ nanoribbon FETs for ion, small-molecule, and oligonucleotide detection where higher surface-to-volume ratios translate to better detection sensitivities.

The Critical Role of Peripheral Targets in Triggering Rapid Neural Effects of Intravenous Cocaine

Direct interaction of cocaine with centrally located monoamine transporters is the primary mechanism underlying its reinforcing properties. It is also often assumed that this drug action is responsible for all the physiological and behavioral effects of this drug. The goal of this review is to challenge this basic mechanism and demonstrate the importance of peripheral actions of cocaine in inducing its initial, rapid neural effects. The use of high-resolution electrophysiological, neurochemical and physiological techniques revealed that the effects of intravenous cocaine at behaviorally relevant doses are exceptionally rapid and transient correlating with strong, quick, and transient increases in blood cocaine levels. Some of these effects are mimicked by cocaine-methiodide, a cocaine analog that cannot cross the blood-brain barrier and they are resistant to dopamine (DA) receptor blockade. Therefore, it appears that rapid neural effects of cocaine result from its direct interaction with receptive sites on afferents of sensory nerves densely innervating blood vessels. This interaction creates a rapid neural signal to the CNS that results in generalized neural activation and subsequent changes in different physiological parameters. This drug's action appears to be independent from cocaine's action on central neurons, which requires a definite time to occur and induce neural and physiological effects with longer latencies and durations. The co-existence in the same drug on two timely distinct actions with their subsequent interaction in the CNS could explain consistent changes in physiological and behavioral effects of cocaine following their repeated use, playing a role in the development of drug-seeking and drug-taking behavior.

Recent advances in neurotechnologies with broad potential for neuroscience research

Interest in deciphering the fundamental mechanisms and processes of the human mind represents a central driving force in modern neuroscience research. Activities in support of this goal rely on advanced methodologies and engineering systems that are capable of interrogating and stimulating neural pathways, from single cells in small networks to interconnections that span the entire brain. Recent research establishes the foundations for a broad range of creative neurotechnologies that enable unique modes of operation in this context. This review focuses on those systems with proven utility in animal model studies and with levels of technical maturity that suggest a potential for broad deployment to the neuroscience community in the relatively near future. We include a brief summary of existing and emerging neuroscience techniques, as background for a primary focus on device technologies that address associated opportunities in electrical, optical and microfluidic neural interfaces, some with multimodal capabilities. Examples of the use of these technologies in recent neuroscience studies illustrate their practical value. The vibrancy of the engineering science associated with these platforms, the interdisciplinary nature of this field of research and its relevance to grand challenges in the treatment of neurological disorders motivate continued growth of this area of study.

Using Aptamers as a Novel Method for Determining GnRH/LH Pulsatility

Aptamers are a novel technology enabling the continuous measurement of analytes in blood and other body compartments, without the need for repeated sampling and the associated reagent costs of traditional antibody-based methodologies. Aptamers are short single-stranded synthetic RNA or DNA that recognise and bind to specific targets. The conformational changes that can occur upon aptamer–ligand binding are transformed into chemical, fluorescent, colour changes and other readouts. Aptamers have been developed to detect and measure a variety of targets in vitro and in vivo. Gonadotropin-releasing hormone (GnRH) is a pulsatile hypothalamic hormone that is essential for normal fertility but difficult to measure in the peripheral circulation. However, pulsatile GnRH release results in pulsatile luteinizing hormone (LH) release from the pituitary gland. As such, LH pulsatility is the clinical gold standard method to determine GnRH pulsatility in humans. Aptamers have recently been shown to successfully bind to and measure GnRH and LH, and this review will focus on this specific area. However, due to the adaptability of aptamers, and their suitability for incorporation into portable devices, aptamer-based technology is likely to be used more widely in the future.

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.

Electrochemical Studies of Cation Condensation-Induced Collapse of Surface-Bound DNA

In this paper, we demonstrate the ability to control and electrochemically monitor nucleic acid conformation by inducing collapse of short, surface-bound nucleotides (7-28 nucleotides). More specifically, we monitored changes in a 5'-electrode-bound DNA structure via changes in the faradaic current related to the reduction/oxidation of a 3'-terminal-appended redox molecule. Reversible DNA collapse was induced by cation condensation achieved by either reducing the dielectric permittivity of the interrogation solution or by the addition of multivalent cations such as the polyamine spermidine (3⁺). Additionally, we find that while the change in electrochemical signal associated with surface bound DNA collapse is dependent on nucleic acid length and surface packing density, the solution conditions (e.g., dielectric permittivity) required for collapse remain constant. As such, we find that collapse of the short DNA strands occurs when the effective charge of the DNA backbone is ∼73-89% neutralized by cations in solution/buffer, according to Manning's theory on cation condensation. This work provides new insight into the structure function relationship of surface-bound nucleic acids and how this is manifested in electrochemical signaling.

Direct in Vivo Electrochemical Detection of Resting Dopamine Using Poly(3,4-ethylenedioxythiophene)/Carbon Nanotube Functionalized Microelectrodes

Dopamine (DA) is a monoamine neurotransmitter responsible for the maintenance of a variety of vital life functions. In vivo DA signaling occurs over multiple time scales, from subsecond phasic release due to dopamine neuron firing to tonic release responsible for long-term DA concentration changes over minutes to hours. Due to the complex, multifaceted nature of DA signaling, analytical sensing technology must be capable of recording DA from multiple locations and over multiple time scales. Decades of research has focused on improving in vivo detection capabilities for subsecond phasic DA, but the accurate detection of absolute resting DA levels in real time has proven challenging. We have developed a poly(3,4-ethylenedioxythiophene) (PEDOT)-based nanocomposite coating that exhibits excellent DA sensing capabilities for resting DA. PEDOT/functionalized carbon nanotube (PEDOT/CNT)-coated carbon fiber microelectrodes (CFEs) are capable of directly measuring resting DA using square wave voltammetry (SWV) with high sensitivity and selectivity. Incorporation of a PEDOT/CNT coating significantly increases the sensitivity for the detection of resting DA by a factor of 422. SWV measurements performed at PEDOT/CNT-functionalized CFEs implanted in the rat dorsal striatum reveal the absolute basal DA concentration to be 82 ± 6 nM. Systemic administration of the dopamine transporter inhibitor nomifensine increases resting DA to a maximum 207 ± 16 nM at 28 ± 2 min following injection. PEDOT/CNT was also functionalized onto individual gold electrode sites along silicon microelectrode arrays (MEAs) to produce a multisite DA sensing electrode. MEA implantation allows for the quantification of basal DA from different brain regions with excellent spatial resolution. SWV detection paired with PEDOT/CNT functionalization is highly adaptable and shows great promise for tonic DA detection with high spatial and temporal resolution.

Facile Synthesis of a 3,4-Ethylene-Dioxythiophene (EDOT) Derivative for Ease of Bio-Functionalization of the Conducting Polymer PEDOT

In the pursuit of conducting polymer based bio-functional devices, a cost-effective and high yield synthesis method for a versatile monomer is desired. We report here a new synthesis strategy for a versatile monomer 2-methylene-2,3-dihydrothieno (3,4-b) (1,4) dioxine, or 3,4-ethylenedioxythiophene with a exomethylene side group (EDOT-EM). Compared to the previously reported synthesis route, the new strategy uses less steps, with faster reaction rate, and higher yield. The presence of EM group opens up endless possibility for derivatization via either hydro-alkoxy addition or thiol-ene click chemistry. EDOT-EM could be polymerized into stable and low impedance PEDOT-EM polymer using electro-polymerization method on different conducting substrates at both macro and micro scales. Facile post-functionalization of PEDOT-EM with molecules of varying size and functionality (from small molecules to DNAs and proteins) was achieved. The new synthetic route of EDOT-EM and the ease of post-functionalization of PEDOT-EM will greatly accelerate the use of conducting polymer in a broad range of organic electronics and bioelectronics applications.

The Glutamatergic Postrhinal Cortex-Ventrolateral Orbitofrontal Cortex Pathway Regulates Spatial Memory Retrieval

A deficit in spatial memory has been taken as an early predictor of Alzheimer's disease (AD) or mild cognitive impairment (MCI). The uncinate fasciculus (UF) is a long-range white-matter tract that connects the anterior temporal lobe with the orbitofrontal cortex (OFC) in primates. Previous studies have shown that the UF impairment associated with spatial memory deficits may be an important pathological change in aging and AD, but its exact role in spatial memory is not well understood. The pathway arising from the postrhinal cortex (POR) and projecting to the ventrolateral orbitofrontal cortex (vlOFC) performs most of the functions of the UF in rodents. Although the literature suggests an association between spatial memory and the regions connected by the POR-vlOFC pathway, the function of the pathway in spatial memory is relatively unknown. To further illuminate the function of the UF in spatial memory, we dissected the POR-vlOFC pathway in mice. We determined that the POR-vlOFC pathway is a glutamatergic structure, and that glutamatergic neurons in the POR regulate spatial memory retrieval. We also demonstrated that the POR-vlOFC pathway specifically transmits spatial information to participate in memory retrieval. These findings provide a deeper understanding of UF function and dysfunction related to disorders of memory, as in MCI and AD.

An in situ assembly strategy for the construction of a sensitive and reusable electrochemical aptasensor

In situ assembly of an electroactive AuNPs–Cu²⁺–l-cysteine tag was applied for the construction of a sensitive and reusable aptasensor.

Enhancing surface immobilization of bioactive molecules via a silica nanoparticle based coating

Surface modification is of significant interest in biomaterials, biosensors, and device biocompatibility. Immobilization of bioactive or biomimetic molecules is a common method of disguising a foreign body as host tissue to decrease the foreign body response (FBR) and/or increase device-tissue integration. For example, in neural interfacing devices, immobilization of L1, a neuron-specific adhesion molecule, has been shown to increase neuron adhesion and reduce inflammatory gliosis on and around the implants. However, the activity of modified surfaces is limited by the relatively low concentration of the immobilized component, in part due to the low surface area of flat surfaces available for modification. In this work, we demonstrate a novel method for increasing the device surface area by attaching a layer of thiolated silica nanoparticles (TNPs). This coating method results in an almost two-fold increase in the immobilized L1 protein. L1 immobilized nanotextured surfaces showed a 100% increase in neurite outgrowth than smooth L1 immobilized surfaces without increasing the adhesion of astrocytes in vitro. The increased bioactivity observed in the cell assay was determined to be mainly due to the higher protein surface density, not the increase in surface roughness. In addition, we tested immobilization of a superoxide dismutase mimic (SODm) on smooth and roughened substrates. The SODm immobilized rough surfaces demonstrated an increase of 145% in superoxide scavenging activity compared to chemically matched smooth surfaces. These results not only show promise in improving biomimetic coating for neural implants, but may also improve surface immobilization efficacy in other fields such as catalysts, protein purification, sensors, and tissue engineering devices.

Two-Dimensional Zirconium-Based Metal-Organic Framework Nanosheet Composites Embedded with Au Nanoclusters: A Highly Sensitive Electrochemical Aptasensor toward Detecting Cocaine

Two-dimensional (2D) zirconium-based metal-organic framework nanosheets embedded with Au nanoclusters (denoted as 2D AuNCs@521-MOF) were prepared via a one-pot method under mild conditions. The optimized 2D AuNCs@521-MOF nanosheets not only possessed high specific surface area, physicochemical stability, and good electrochemical activity but also exhibited strong bioaffinity toward biomolecule-bearing phosphate groups. Consequently, a large amount of cocaine aptamer strands can be immobilized onto the substrate modified by 2D AuNCs@521-MOF nanosheet, further leading to the formation of a constructed biosensitive platform, which can be used to successfully detect cocaine through the specific binding interactions between cocaine and aptamer strands. The results demonstrated that the 2D AuNCs@521-MOF-based aptasensor had high sensitivity for detecting cocaine within the broad concentration range of 0.001-1.0 ng·mL^-1 and the low limit of detection of 1.29 pM (0.44 pg·mL^-1) and 2.22 pM (0.75 pg·mL^-1) as determined by electrochemical impedance spectroscopy and differential pulse voltammetry, respectively. As expected, with the advantages of high selectivity, repeatability, stability, and simple operation, this new strategy is believed to exhibit great potential for simple and convenient detection of cocaine.