Electrochemical nanoprobes for the chemical detection of neurotransmitters

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.

Design of optoelectrodes for the remote imaging of cells and in situ electrochemical detection of neurosecretory events

Optical fibers have opened avenues for remote imaging, bioanalyses and recently optogenetics. Besides, miniaturized electrochemical sensors have offered new opportunities in sensing directly redox neurotransmitters. The combination of both optical and electrochemical approaches was usually performed on the platform of microscopes or within microsystems. In this work, we developed optoelectrodes which features merge the advantages of both optical fibers and microelectrodes. Optical fiber bundles were modified at one of their extremity by a transparent ITO deposit. The electrochemical responses of these ITO-modified bundles were characterized for the detection of dopamine, epinephrine and norepinephrine. The analytical performances of the optoelectrodes were equivalent to the ones reported for carbon microelectrodes. The remote imaging of model neurosecretory PC12 cells by optoelectrodes was performed upon cell-staining with common fluorescent dyes: acridine orange and calcein-AM. An optoelectrode placed by micromanipulation at a few micrometers-distance from the cells offered remote images with single cell resolution. Finally, in situ electrochemical sensing was demonstrated by additions of K⁺-secretagogue solutions near PC12 cells under observation, leading to exocytotic events detected as amperometric spikes at the ITO surface. Such dual sensors should pave the way for in vivo remote imaging, optogenetic stimulation, and simultaneous detection of neurosecretory activities.

Carbon nanospike coated nanoelectrodes for measurements of neurotransmitters

Carbon nanoelectrodes enable the detection of neurotransmitters at the level of single cells, vesicles, synapses and small brain structures. Previously, the etching of carbon fibers and 3D printing based on direct laser writing have been used to fabricate carbon nanoelectrodes, but these methods lack the ability of mass manufacturing. In this paper, we mass fabricate carbon nanoelectrodes by growing carbon nanospikes (CNSs) on metal wires. CNSs have a short, dense and defect-rich surface that produces remarkable electrochemical properties, and they can be mass fabricated on almost any substrate without using catalysts. Tungsten wires and niobium wires were electrochemically etched in batch to form sub micrometer sized tips, and a layer of CNSs was grown on the metal wires using plasma-enhanced chemical vapor deposition (PE-CVD). The thickness of the CNS layer was controlled by the deposition time, and a thin layer of CNSs can effectively cover the entire metal surface while maintaining the tip size within the sub micrometer scale. The etched tungsten wires produced tapered conical nanotips, while the etched niobium wires were long and thin. Both showed excellent sensitivity for the detection of outer sphere ruthenium hexamine and the inner sphere test compound ferricyanide. The CNS nanosensors were used for the measurement of dopamine, serotonin, ascorbic acid and DOPAC with fast-scan cyclic voltammetry. The CNS nanoelectrodes had a large surface area and numerous defect sites, which improved the sensitivity, electron transfer kinetics and adsorption. Finally, the CNS nanoelectrodes were compared with other nanoelectrode fabrication methods, including flame etching, 3D printing, and nanopipettes, which are slower to make and more difficult for mass fabrication. Thus, CNS nanoelectrodes are a promising strategy for the mass fabrication of nanoelectrode sensors for neurotransmitters.

Detection of Acetylcholine at Nanoscale NPOE/Water Liquid/Liquid Interface Electrodes

The interface between two immiscible electrolyte solutions (ITIES) has become a very powerful analytical platform for sensing a diverse range of chemicals (e.g., metal ions and neurotransmitters) with the advantage of being able to detect non-redox electroactive species. The ITIES is formed between organic and aqueous phases. Organic solvent identity is crucial to the detection characteristics of the ITIES [half-wave transfer potential (E_1/2), potential window range, limit of detection, transfer coefficient (α), standard heterogeneous ion-transfer rate constant (k⁰), etc.]. Here, we demonstrated, for the first time at the nanoscale, the detection characteristics of the NPOE/water ITIES. Linear detection of the diffusion-limited current at different concentrations of acetylcholine (ACh) was demonstrated with cyclic voltammetry (CV) and i-t amperometry. The E_1/2 of ACh transfer at the NPOE/water nanoITIES was -0.342 ± 0.009 V versus the E_1/2 of tetrabutylammonium (TBA⁺). The limit of detection of ACh at the NPOE/water nanoITIES was 37.1 ± 1.5 μM for an electrode with a radius of ∼127 nm. We also determined the ion-transfer kinetics parameters, α and k⁰, of TBA⁺ at the NPOE/water nanoITIES by fitting theoretical cyclic voltammograms to experimental voltammograms. This work lays the basis for future cellular studies using ACh detection at the nanoscale and for studies to detect other analytes. The NPOE/water ITIES offers a potential window distinct from that of the 1,2-dichloroethane (DCE)/water ITIES. This unique potential window would offer the ability to detect analytes that are not easily detected at the DCE/water ITIES.

Nanoneedle-Based Materials for Intracellular Studies

Nanoneedles, defined as high aspect ratio structures with tip diameters of 5 to approximately 500 nm, are uniquely able to interface with the interior of living cells. Their nanoscale dimensions mean that they are able to penetrate the plasma membrane with minimal disruption of normal cellular functions, allowing researchers to probe the intracellular space and deliver or extract material from individual cells. In the last decade, a variety of strategies have been developed using nanoneedles, either singly or as arrays, to investigate the biology of cancer cells in vitro and in vivo. These include hollow nanoneedles for soluble probe delivery, nanocapillaries for single-cell biopsy, nano-AFM for direct physical measurements of cytosolic proteins, and a wide range of fluorescent and electrochemical nanosensors for analyte detection. Nanofabrication has improved to the point that nanobiosensors can detect individual vesicles inside the cytoplasm, delineate tumor margins based on intracellular enzyme activity, and measure changes in cell metabolism almost in real time. While most of these applications are currently in the proof-of-concept stage, nanoneedle technology is poised to offer cancer biologists a powerful new set of tools for probing cells with unprecedented spatial and temporal resolution.

"Nano": An Emerging Avenue in Electrochemical Detection of Neurotransmitters

The growing importance of nanomaterials toward the detection of neurotransmitter molecules has been chronicled in this review. Neurotransmitters (NTs) are chemicals that serve as messengers in synaptic transmission and are key players in brain functions. Abnormal levels of NTs are associated with numerous psychotic and neurodegenerative diseases. Therefore, their sensitive and robust detection is of great significance in clinical diagnostics. For more than three decades, electrochemical sensors have made a mark toward clinical detection of NTs. The superiority of these electrochemical sensors lies in their ability to enable sensitive, simple, rapid, and selective determination of analyte molecules while remaining relatively inexpensive. Additionally, these sensors are capable of being integrated in robust, portable, and miniaturized devices to establish point-of-care diagnostic platforms. Nanomaterials have emerged as promising materials with significant implications for electrochemical sensing due to their inherent capability to achieve high surface coverage, superior sensitivity, and rapid response in addition to simple device architecture and miniaturization. Considering the enormous significance of the levels of NTs in biological systems and the advances in sensing ushered in with the integration of nanotechnology in electrochemistry, the analysis of NTs by employing nanomaterials as interface materials in various matrices has emerged as an active area of research. This review explores the advancements made in the field of electrochemical sensors for the sensitive and selective determination of NTs which have been described in the past two decades with a distinctive focus on extremely innovative attributes introduced by nanotechnology.

Quantitative Analysis of Redox-Inactive Ions by AC Voltammetry at a Polarized Interface between Two Immiscible Electrolyte Solutions

The interface between two immiscible electrolyte solutions (ITIES) is ideally suited to detect redox-inactive ions by their ion transfer. Such electroanalysis, based on the Nernst-Donnan equation, has been predominantly performed using amperometry, cyclic voltammetry, or differential pulse voltammetry. Here, we introduce a new electroanalytical method based on alternating-current (AC) voltammetry with inherent advantages over traditional approaches such as avoidance of positive feedback iR compensation, a major issue for liquid|liquid electrochemical cells containing resistive organic media and interfacial areas in the cm² and mm² range. A theoretical background outlining the generation of the analytical signal is provided and based on extracting the component that depends on the Warburg impedance from the total impedance. The quantitative detection of a series of model redox-inactive tetraalkylammonium cations is demonstrated, with evidence provided of the transient adsorption of these cations at the interface during the course of ion transfer. Since ion transfer is diffusion-limited, by changing the voltage excitation frequency during AC voltammetry, the intensity of the Faradaic response can be enhanced at low frequencies (1 Hz) or made to disappear completely at higher frequencies (99 Hz). The latter produces an AC voltammogram equivalent to a "blank" measurement in the absence of analyte and is ideal for background subtraction. Therefore, major opportunities exist for the sensitive detection of ionic analyte when a "blank" measurement in the absence of analyte is impossible. This approach is particularly useful to deconvolute signals related to reversible electrochemical reactions from those due to irreversible processes, which do not give AC signals.

Nanoelectrochemistry in the study of single-cell signaling

Label-free biosensing has been the dream of scientists and biotechnologists as reported by Vollmer and Arnold (Nat Methods 5:591-596, 2008). The ability of examining living cells is crucial to cell biology as noted by Fang (Int J Electrochem 2011:460850, 2011). Chemical measurement with electrodes is label-free and has demonstrated capability of studying living cells. In recent years, nanoelectrodes of different functionality have been developed. These nanometer-sized electrodes, coupled with scanning electrochemical microscopy (SECM), have further enabled nanometer spatial resolution study in aqueous environments. Developments in the field of nanoelectrochemistry have allowed measurement of signaling species at single cells, contributing to better understanding of cell biology. Leading studies using nanoelectrochemistry of a variety of cellular signaling molecules, including redox-active neurotransmitter (e.g., dopamine), non-redox-active neurotransmitter (e.g., acetylcholine), reactive oxygen species (ROS), and reactive nitrogen species (RNS), are reviewed here.

Mitigating the Effects of Electrode Biofouling-Induced Impedance for Improved Long-Term Electrochemical Measurements In Vivo

Biofouling is a prevalent issue in studies that involve prolonged implantation of electrochemical probes in the brain. In long-term fast-scan cyclic voltammetry (FSCV) studies, biofouling manifests as a shift in the peak oxidative potential of the background signal that worsens over days to weeks, diminishing sensitivity and selectivity to neurotransmitters such as dopamine. Using open circuit potential (OCP) measurements, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), and electrochemical impedance spectroscopy (EIS), we examined the biofouling-induced events that occur due to electrode implantation. We determined that the FSCV background signal shift results from cathodic polarization of the Ag/AgCl-wire reference electrode and increased electrochemical impedance of both the Ag/AgCl-wire reference electrode and carbon-fiber working electrode. These events are likely caused collectively by immune response-induced electrode encapsulation. A headstage utilizing a three-electrode configuration, designed to compensate for the impedance component of biofouling, reduced the FSCV background signal shift in vivo and preserved dopamine sensitivity at artificially increased impedance levels in vitro. In conjunction with a stable reference electrode, this three-electrode configuration will be critical in achieving reliable neurotransmitter detection for the duration of long-term FSCV studies.

Review-Recent Advances in Nanosensors Built with Pre-Pulled Glass Nanopipettes and Their Applications in Chemical and Biological Sensing

Nanosensors built with pre-pulled glass nanopipettes, including bare or chemically modified nanopipettes and fully or partially filled solid nanoelectrodes, have found applications in chemical and biological sensing via resistive-pulse, current rectification, and electrochemical sensing. These nanosensors are easily fabricated and provide advantages through their needle-like geometry with nanometer-sized tips, making them highly sensitive and suitable for local measurements in extremely small samples. The variety in the geometry and layout have extended sensing capabilities. In this review, we will outline the fundamentals in fabrication, modification, and characterization of those pre-pulled glass nanopipette based nanosensors and highlight the most recent progress in their development and applications in real-time monitoring of biological processes, chemical ion sensing, and single entity analysis.

In Vivo Electrochemical Sensors for Neurochemicals: Recent Update

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

Cavity Carbon-Nanopipette Electrodes for Dopamine Detection

Microelectrodes are typically used for neurotransmitter detection, but nanoelectrodes are not because there is a trade-off between spatial resolution and sensitivity that is dependent on surface area. Cavity carbon-nanopipette electrodes (CNPEs), with tip diameters of a few hundred nanometers, have been developed for nanoscale electrochemistry. Here, we characterize the electrochemical performance of CNPEs with fast-scan cyclic voltammetry (FSCV) for the first time. Dopamine detection using cavity CNPEs, with a depth equivalent to a few radii, is compared with that using open-tube CNPEs, an essentially infinite geometry. Open-tube CNPEs have very slow temporal responses that change over time as the liquid rises in the CNPE. However, a cavity CNPE has a fast temporal response to a bolus of dopamine that is not different from that of a traditional carbon-fiber microelectrode. Cavity CNPEs, with tip diameters of 200-400 nm, have high currents because the small cavity traps and increases the local dopamine concentration. The trapping also leads to an FSCV frequency-independent response and the appearance of cyclization peaks that are normally observed only with large concentrations of dopamine. CNPEs have high dopamine selectivity over ascorbic acid (AA) because of the repulsion of AA by the negative electric field at the holding potential and the irreversible redox reaction. In mouse-brain slices, cavity CNPEs detected exogenously applied dopamine, showing they do not clog in tissue. Thus, cavity CNPEs are promising neurochemical sensors that provide spatial resolution on the scale of hundreds of nanometers, which is useful for small model organisms or for locations near specific cells.

Single Synaptic Observation of Cholinergic Neurotransmission on Living Neurons: Concentration and Dynamics

Acetylcholine, the first neurotransmitter identified more than a century ago, plays critical roles in human activities and health; however, its synaptic concentration dynamics have remained unknown. Here, we demonstrate the in situ simultaneous measurements of synaptic cholinergic transmitter concentration and release dynamics. We used nanoscale electroanalytical methods: nanoITIES electrode of 15 nm in radius and nanoresolved scanning electrochemical microscopy (SECM). Time-resolved in situ measurements unveiled information on synaptic acetylcholine concentration and release dynamics of living Aplysia neurons. The measuring technique enabled the quantitative sensing of acetylcholine with negligible interference of other ionic and redox-active species. We measured cholinergic transmitter concentrations very close to the synapse, with values as high as 2.4 mM. We observed diverse synaptic transmitter concentration dynamics consisting of singlet, doublet and multiplet events with a signal-to-noise ratio of 6 to 130. The unprecedented details about synaptic neurotransmission unveiled are instrumental for understanding brain communication and diseases in a way distinctive from extra-synaptic studies.

A high spatiotemporal study of somatic exocytosis with scanning electrochemical microscopy and nanoITIES electrodes

Extra-synaptic exocytosis is an essential component of cellular communication. A knowledge gap exists in the exocytosis of the non-redox active transmitter acetylcholine. Using the nano-interface between two immiscible electrolyte solutions and scanning electrochemical microscopy (SECM), a high resolution spatiotemporal study of acetylcholine exocytosis is shown from an individual neuronal soma. The nanoelectrode was positioned ∼140 nm away from the release sites on the soma using an SECM. The quantitative study enables the obtaining of key information related to cellular communication, including extracellular concentration of the neurotransmitter, cellular permeability, Ca2+ dependence on somatic release, vesicle density, number of molecules released and the release dynamics. Measurements were achieved with a high signal to noise ratio of 6-19. The released neurotransmitter with a concentration of 2.7 (±1.0) μM was detected at the nanoelectrodes with radii of 750 nm to 860 nm.

Fabrication and Use of Nanopipettes in Chemical Analysis

This review summarizes progress in the fabrication, modification, characterization, and applications of nanopipettes since 2010. A brief history of nanopipettes is introduced, and the details of fabrication, modification, and characterization of nanopipettes are provided. Applications of nanopipettes in chemical analysis are the focus in several cases, including recent progress in imaging; in the study of single molecules, single nanoparticles, and single cells; in fundamental investigations of charge transfer (ion and electron) reactions at liquid/liquid interfaces; and as hyphenated techniques combined with other methods to study the mechanisms of complicated electrochemical reactions and to conduct bioanalysis.

GABA Detection with Nano-ITIES Pipet Electrode: A New Mechanism, Water/DCE-Octanoic Acid Interface

Interface between two immiscible electrolyte solutions (ITIES) supported on the orifice of a pipet have become a powerful platform to detect a broad range of analytes. We present here the detection of γ-aminobutyric acid (GABA) with the nanoITIES pipet electrodes for the first time. GABA has a net charge of zero in an aqueous solution at pH ≈ 7, and it has not previously been detected at ITIES. In this work, we demonstrated GABA detection at ITIES in an aqueous solution at pH ≈ 7, where we introduced a novel detection strategy based on "pH modulation from the oil phase". To the best of our knowledge, this is the first report of such. Current increases linearly with increasing concentrations of GABA, ranging from 0.25 mM to 1.0 mM. The measured half-wave transfer potential of GABA is -0.401 ± 0.010 V ( n = 22) vs E1/2,TBA. The measured diffusion coefficient for GABA detection at nanoITIES pipet electrode is 6.09 (±0.58) × 10-10 m2/s ( n = 5). Experimental results indicate that protons generated from octanoic acid dissociation in the oil phase do not come out from the oil phase into the aqueous phase; neither were protons produced in the aqueous phase. NanoITIES pipet electrodes with radii of 320-340 nm were used in the current study. This new strategy and knowledge presented here lays the groundwork for the future development of ITIES pipet electrodes, especially for the detection of electrochemically nonredox active analytes.

Tuning Selectivity of Fluorescent Carbon Nanotube-Based Neurotransmitter Sensors

Detection of neurotransmitters is an analytical challenge and essential to understand neuronal networks in the brain and associated diseases. However, most methods do not provide sufficient spatial, temporal, or chemical resolution. Near-infrared (NIR) fluorescent single-walled carbon nanotubes (SWCNTs) have been used as building blocks for sensors/probes that detect catecholamine neurotransmitters, including dopamine. This approach provides a high spatial and temporal resolution, but it is not understood if these sensors are able to distinguish dopamine from similar catecholamine neurotransmitters, such as epinephrine or norepinephrine. In this work, the organic phase (DNA sequence) around SWCNTs was varied to create sensors with different selectivity and sensitivity for catecholamine neurotransmitters. Most DNA-functionalized SWCNTs responded to catecholamine neurotransmitters, but both dissociation constants (K_d) and limits of detection were highly dependent on functionalization (sequence). K_d values span a range of 2.3 nM (SWCNT-(GC)₁₅ + norepinephrine) to 9.4 μM (SWCNT-(AT)₁₅ + dopamine) and limits of detection are mostly in the single-digit nM regime. Additionally, sensors of different SWCNT chirality show different fluorescence increases. Moreover, certain sensors (e.g., SWCNT-(GT)₁₀) distinguish between different catecholamines, such as dopamine and norepinephrine at low concentrations (50 nM). These results show that SWCNTs functionalized with certain DNA sequences are able to discriminate between catecholamine neurotransmitters or to detect them in the presence of interfering substances of similar structure. Such sensors will be useful to measure and study neurotransmitter signaling in complex biological settings.

Advanced electroanalytical chemistry at nanoelectrodes

Nanoelectrodes, with dimensions below 100 nm, have the advantages of high sensitivity and high spatial resolution. These electrodes have attracted increasing attention in various fields such as single cell analysis, single-molecule detection, single particle characterization and high-resolution imaging. The rapid growth of novel nanoelectrodes and nanoelectrochemical methods brings enormous new opportunities in the field. In this perspective, we discuss the challenges, advances, and opportunities for nanoelectrode fabrication, real-time characterizations and high-performance electrochemical instrumentation.

Focused-Ion-Beam-Milled Carbon Nanoelectrodes for Scanning Electrochemical Microscopy

Nanoscale scanning electrochemical microscopy (SECM) has emerged as a powerful electrochemical method that enables the study of interfacial reactions with unprecedentedly high spatial and kinetic resolution. In this work, we develop carbon nanoprobes with high electrochemical reactivity and well-controlled size and geometry based on chemical vapor deposition of carbon in quartz nanopipets. Carbon-filled nanopipets are milled by focused ion beam (FIB) technology to yield a flat disk tip with a thin quartz sheath as confirmed by transmission electron microscopy. The extremely high electroactivity of FIB-milled carbon nanotips is quantified by enormously high standard electron-transfer rate constants of ≥10 cm/s for Ru(NH3)63+. The tip size and geometry are characterized in electrolyte solutions by SECM approach curve measurements not only to determine inner and outer tip radii of down to ~27 and ~38 nm, respectively, but also to ensure the absence of a conductive carbon layer on the outer wall. In addition, FIB-milled carbon nanotips reveal the limited conductivity of ~100 nm-thick gold films under nanoscale mass-transport conditions. Importantly, carbon nanotips must be protected from electrostatic damage to enable reliable and quantitative nanoelectrochemical measurements.

Electroanalytical Ventures at Nanoscale Interfaces Between Immiscible Liquids

Ion transfer at the interface between immiscible electrolyte solutions offers many benefits to analytical chemistry, including the ability to detect nonredox active ionized analytes, to detect ions whose redox electrochemistry is accompanied by complications, and to separate ions based on electrocontrolled partition. Nanoscale miniaturization of such interfaces brings the benefits of enhanced mass transport, which in turn leads to improved analytical performance in areas such as sensitivity and limits of detection. This review discusses the development of such nanoscale interfaces between immiscible liquids and examines the analytical advances that have been made to date, including prospects for trace detection of ion concentrations.

Electrochemical Detection of Dopamine via Assisted Ion Transfer at Nanopipet Electrode Using Cyclic Voltammetry

We present here the detection of dopamine (DA) at nanopipet electrodes with radii of hundreds of nanometers ranging from 160 nm to 480 nm. Dibenzo-18-crown-6 (DB18C6) was employed as an ionophore to facilitate DA transfer, resulting in a half-wave transfer potential, E1/2, DA, of -0.322 (±0.020) V vs. E1/2, TBA. Well-defined steady-state sigmoidal cyclic voltammograms were observed for the transfer of DA. High resolution scanning electron microscopy was used to measure the size and taper angle of the nanopipet electrodes. The detection is linear with concentration of DA ranging from 0.25 mM to 2 mM; calculated diffusion coefficient at nanopipet electrodes with above mentioned sizes is 4.87 (±0.28) × 10-10 m2/s. The effect of the common interferent ascorbic acid on DA detection with nanopipet electrodes was evaluated, where DA detection still shows linear behavior with well-defined sigmoidal CVs with E1/2, DA being -0.328 (±0.029) V vs. E1/2, TBA. The diffusion coefficient for DA transfer in MgCl2 with the presence of 2 mM AA was measured to be 1.93 (±0.59) × 10-10 m2/s on nanoelectrodes with radii from 161 nm to 263 nm, but the physiological concentration of 0.1 mM AA had no effect on DA's diffusion coefficient.