Simultaneous study of subcellular exocytosis with individually addressable multiple microelectrodes

Nanoelectrochemical quantification of single-cell metabolism

At the most fundamental level, the behavior of tissue is governed by the activity of its single cells. A detailed examination of single-cell biology is necessary in order to gain a deeper understanding of disease progression. While single-cell genomics and transcriptomics are mature due to robust amplification strategies, the metabolome is difficult to quantify. Nanoelectrochemical techniques stand poised to quantify single-cell metabolism as a result of the fabrication of nanoelectrodes, which allow one to make intracellular electrochemical measurements. This article is concerned with intracellular nanoelectrochemistry, focusing on the sensitive and selective quantification of various metabolites within a single, living cell. We will review the strong literature behind this field, discuss the potential deleterious effects of passing charge inside cells, and provide future outlooks for this promising avenue of inquiry. We also present a mathematical relationship based on Faraday's Law and bulk electrolysis theory to examine the consumption of analyte within a cell due to passing charge at the nanotip.Graphical abstract.

Electrochemistry of Single-Vesicle Events

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

Review Article: Tools and trends for probing brain neurochemistry

The brain is composed of complex neuronal networks that interact on spatial and temporal scales that span several orders of magnitude. Uncovering how this circuitry gives rise to multifaceted phenomena such as perception, memory, and behavior remains one of the grand challenges in science today. A wide range of investigative methods have been developed to delve deeper into the inner workings of the brain, spanning the realms of molecular biology, genetics, chemistry, optics, and engineering, thereby forming a nexus of discovery that has accelerated our understanding of the brain. Whereas neuronal electrical excitability is a hallmark property of neurons, chemical signaling between neurons-mediated by hundreds of neurotransmitters, neuromodulators, hormones, and other signaling molecules-is equally important, but far more elusive in its regulation of brain function for motor control, learning, and behavior. To date, the brain's neurochemical state has been interrogated using classical tools borrowed from analytical chemistry, such as liquid chromatography and amperometry, and more recently, newly developed fluorescent sensors. Here, the authors review advances in the development of functional fluorescent probes that are beginning to expand their understanding of the neurochemical basis of brain function alongside device-based analytical tools that have already made extensive contributions to the field. The emphasis herein is on the paradigms of probe and device development, which follow certain design principles unique to the interrogation of brain chemistry.

Review: New insights into optimizing chemical and 3D surface structures of carbon electrodes for neurotransmitter detection

The carbon-fiber microelectrode has been used for decades as a neurotransmitter sensor. Recently, new strategies have been developed for making carbon electrodes, including using carbon nanomaterials or pyrolyzing photoresist etched by nanolithography or 3D printing. This review summarizes how chemical and 3D surface structures of new carbon electrodes are optimized for neurotransmitter detection. There are effects of the chemical structure that are advantageous and nanomaterials are used ranging from carbon nanotube (CNT) to graphene to nanodiamond. Functionalization of these materials promotes surface oxide groups that adsorb dopamine and dopants introduce defect sites good for electron transfer. Polymer coatings such as poly(3,4-ethylenedioxythiophene) (PEDOT) or Nafion also enhance the selectivity, particularly for dopamine over ascorbic acid. Changing the 3D surface structure of an electrode increases current by adding more surface area. If the surface structure has roughness or pores on the micron scale, the electrode also acts as a thin layer cell, momentarily trapping the analyte for redox cycling. Vertically-aligned CNTs as well as lithographically-made or 3D printed pillar arrays act as thin layer cells, producing more reversible cyclic voltammograms. A better understanding of how chemical and surface structure affects electrochemistry enables rational design of electrodes. New carbon electrodes are being tested in vivo and strategies to reduce biofouling are being developed. Future studies should test the robustness for long term implantation, explore electrochemical properties of neurotransmitters beyond dopamine, and combine optimized chemical and physical structures for real-time monitoring of neurotransmitters.

Label-free imaging of neurotransmitters in live brain tissue by multi-photon ultraviolet microscopy

Visualizing small biomolecules in living cells remains a difficult challenge. Neurotransmitters provide one of the most frustrating examples of this difficulty, as our understanding of signaling in the brain critically depends on our ability to follow the neurotransmitter traffic. Last two decades have seen considerable progress in probing some of the neurotransmitters, e.g. by using false neurotransmitter mimics, chemical labeling techniques, or direct fluorescence imaging. Direct imaging harnesses the weak UV fluorescence of monoamines, which are some of the most important neurotransmitters controlling mood, memory, appetite, and learning. Here we describe the progress in imaging of these molecules using the least toxic direct excitation route found so far, namely multi-photon (MP) imaging. MP imaging of serotonin, and more recently that of dopamine, has allowed researchers to determine the location of the vesicles, follow their intracellular dynamics, probe their content, and monitor their release. Recent developments have even allowed ratiometric quantitation of the vesicular content. This review shows that MP ultraviolet (MP-UV) microscopy is an effective but underutilized method for imaging monoamine neurotransmitters in neurones and brain tissue.

Electrochemical measurement of quantal exocytosis using microchips

Carbon-fiber electrodes (CFEs) are the gold standard for quantifying the release of oxidizable neurotransmitters from single vesicles and single cells. Over the last 15 years, microfabricated devices have emerged as alternatives to CFEs that offer the possibility of higher throughput, subcellular spatial resolution of exocytosis, and integration with other techniques for probing exocytosis including microfluidic cell handling and solution exchange, optical imaging and stimulation, and electrophysiological recording and stimulation. Here we review progress in developing electrochemical electrode devices capable of resolving quantal exocytosis that are fabricated using photolithography.

High-resolution imaging of cellular dopamine efflux using a fluorescent nanosensor array

Intercellular communication via chemical signaling proceeds with both spatial and temporal components, but analytical tools, such as microfabricated electrodes, have been limited to just a few probes per cell. In this work, we use a nonphotobleaching fluorescent nanosensor array based on single-walled carbon nanotubes (SWCNTs) rendered selective to dopamine to study its release from PC12 neuroprogenitor cells at a resolution exceeding 20,000 sensors per cell. This allows the spatial and temporal dynamics of dopamine release, following K⁺ stimulation, to be measured at exceedingly high resolution. We observe localized, unlabeled release sites of dopamine spanning 100 ms to seconds that correlate with protrusions but not predominately the positive curvature associated with the tips of cellular protrusions as intuitively expected. The results illustrate how directionality of chemical signaling is shaped by membrane morphology, and highlight the advantages of nanosensor arrays that can provide high spatial and temporal resolution of chemical signaling.

Microfabricated, Massive Electrochemical Arrays of Uniform Ultramicroelectrodes

We report the preparation and electrochemical characterization of massive electrochemical arrays containing as many as 110,000 highly uniform ultramicroelectrodes (UMEs). These arrays were microfabricated using conventional photolithography techniques on a gold-coated silicon chip in a simple three-step method. Photoresist polymer was used as an effective insulating matrix to define 2 μm, 3 μm, and 4 μm diameter circular UMEs across a 1 × 1 mm² area. The UME arrays are high uniform and contain tens of thousands of active disk-shape UMEs slightly recessed in thin films of photoresist. These arrays were tested with cyclic voltammetry and copper electrodeposition to assess the adhesion of photoresist to the gold surface as well as to examine their electrochemical activity. Numerical simulations were performed to further validate their electrochemical response. These UME arrays can be a useful platform for fundamental understanding molecular transport in uniform electrochemical arrays and designing highly-sensitive electroanalytical sensors.

Lithographic Microfabrication of a 16-Electrode Array on a Probe Tip for High Spatial Resolution Electrochemical Localization of Exocytosis

We report the lithographic microfabrication of a movable thin film microelectrode array (MEA) probe consisting of 16 platinum band electrodes placed on top of a supporting borosilicate glass substrate. These 1.2 μm wide electrodes were tightly packed and positioned parallel in two opposite rows within a 20 μm × 25 μm square area and with a distance less than 10 μm from the edge of the glass substrate. We demonstrate the ability to control and place the probe in close proximity to the surface of adherent bovine chromaffin cells and to amperometrically record single exocytosis release events with high spatiotemporal resolution. The two-dimensional position of single exocytotic events occurring in the center gap area separating the two rows of MEA band electrodes and that were codetected by electrodes in both rows was determined by analysis of the fractional detection of catecholamine released between electrodes and exploiting random walk simulations. Hence, two-dimensional electrochemical imaging recording of exocytosis release between the electrodes within this area was achieved. Similarly, by modeling the current spikes codetected by parallel adjacent band electrodes positioned in the same electrode row, a one-dimensional imaging of exocytosis with submicrometer resolution was accomplished within the area. The one- and two-dimensional electrochemical imaging using the MEA probe allowed for high spatial resolution of exocytosis activity and revealed heterogeneous release of catecholamine at the chromaffin cell surface.