Carbon nanospike coated nanoelectrodes for measurements of neurotransmitters

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

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

Size-Dependent Electrochemistry of Laser-Induced Graphene Electrodes

Laser-induced graphene (LIG) electrodes have become popular for electrochemical sensor fabrication due to their simplicity for batch production without the use of reagents. The high surface area and favorable electrocatalytic properties also enable the design of small electrochemical devices while retaining the desired electrochemical performance. In this work, we systematically investigated the effect of LIG working electrode size, from 0.8 mm to 4.0 mm diameter, on their electrochemical properties, since it has been widely assumed that the electrochemistry of LIG electrodes is independent of size above the microelectrode size regime. The background and faradaic current from cyclic voltammetry (CV) of an outer-sphere redox probe [Ru(NH3)6]3+ showed that smaller LIG electrodes had a higher electrode roughness factor and electroactive surface ratio than those of the larger electrodes. Moreover, CV of the surface-sensitive redox probes [Fe(CN)6]3- and dopamine revealed that smaller electrodes exhibited better electrocatalytic properties, with enhanced electron transfer kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy showed that the physical and chemical surface structure were different at the electrode center versus the edges, so the electrochemical properties of the smaller electrodes were improved by having rougher surface and more density of the graphitic edge planes, and more oxide-containing groups, leading to better electrochemistry. The difference could be explained by the different photothermal reaction time from the laser scribing process that causes different stable carbon morphology to form on the polymer surface. Our results give a new insight on relationships between surface structure and electrochemistry of LIG electrodes and are useful for designing miniaturized electrochemical devices.

3D-Printed Carbon Nanoneedle Electrodes for Dopamine Detection in Drosophila

In vivo electrochemistry in small brain regions or synapses requires nanoelectrodes with long straight tips for submicron scale measurements. Nanoelectrodes can be fabricated using a Nanoscribe two-photon printer, but annealed tips curl if they are long and thin. We propose a new pulling-force strategy to fabricate a straight carbon nanoneedle structure. A micron-width bridge is printed between two blocks. The annealed structure shrinks during pyrolysis, and the blocks create a pulling force to form a long, thin, and straight carbon bridge. Parameterization study and COMSOL modeling indicate changes in the block size, bridge size and length affect the pulling force and bridge shrinkage. Electrodes were printed on niobium wires, insulated with aluminum oxide, and the bridge cut with focused ion beam (FIB) to expose the nanoneedle tip. Annealed needle diameters ranged from 400 nm to 5.25 μm and length varied from 50.5 μm to 146 μm. The electrochemical properties are similar to glassy carbon, with good performance for dopamine detection with fast-scan cyclic voltammetry. Nanoelectrodes enable biological applications, such as dopamine detection in a specific Drosophila brain region. Long and thin nanoneedles are generally useful for other applications such as cellular sensing, drug delivery, or gas sensing.

Hairpin DNA-based electrochemical amplification strategy for miRNA sensing by using single gold nanoelectrodes

A new sensor has been developed to detect miRNA-15 using nanoelectrodes and a hairpin DNA-based electrochemical amplification technique. By utilizing a complex DNA cylinder connected with hairpin DNA1, the sensor is able to absorb more methylene blue (MB) than simple double-stranded DNA. Another hairpin DNA2 is modified on an Au nanoelectrode surface and, when miRNA-15 is introduced, it triggers a chain reaction. This reaction unlocks two hairpins alternatively to polymerize into a complex structure that attaches more MB. The miRNA-15 is then replaced by DNA1 due to strand displacement reactions and continues to react with the next DNA2 to achieve circular amplification. The electrochemical signal from MB oxidation has a linear relationship with the miRNA-15 concentrations, making it possible to detect miRNA-15. Moreover, this method can be readily adapted for the detection of various other miRNA species. The newly devised nanosensor holds promising applications for the in vivo detection of miRNA-15 within biological systems, which is achieved by leveraging the advantageous characteristics of nanoelectrodes, including their low resistance-capacitance time constant, rapid mass transfer kinetics, and small diameter.

Variable Nanoelectrode at the Air/Water Interface by Hydrogel-Integrated Atomic Force Microscopy Electrochemical Platform

A nanoelectrode with a controllable area was developed using commercial atomic force microscopy and a hydrogel. Although tremendous advantages of small electrodes from micrometer scale down to nanometer scale have been previously reported for a wide range of applications, precise and high-throughput fabrication remains an obstacle. In this work, the set-point feedback current in a modified scanning ionic conductance microscopy system controlled the formation of electrodes with a nanometer-sized area by contact between the boron-doped diamond (BDD) tip and the agarose hydrogel. The modulation of the electroactive area of the BDD-coated nanoelectrode in the hydrogel was successively investigated by the finite element method and cyclic voltammetry with the use of a redox-contained hydrogel. Moreover, this nanoelectrode enables the simultaneous imaging of both the topography and electrochemical activity of a polymeric microparticle embedded in a hydrogel.

Accelerating the development of implantable neurochemical biosensors by using existing clinically applied depth electrodes

In this study, an implantable stereo-electroencephalography (sEEG) depth electrode was functionalised with an enzyme coating for enzyme-based biosensing of glucose and L-glutamate. This was done because personalised medicine could benefit from active real-time neurochemical monitoring on small spatial and temporal scales to further understand and treat neurological disorders. To achieve this, the sEEG depth electrode was characterised using cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS) using several electrochemical redox mediators (potassium ferri/ferrocyanide, ruthenium hexamine chloride, and dopamine). To improve performance, the Pt sensors on the sEEG depth electrode were coated with platinum black and a crosslinked gelatin-enzyme film to enable enzymatic biosensing. This characterisation work showed that producing a useable electrode with a good electrochemical response showing the expected behaviour for a platinum electrode was possible. Coating with Pt black improved the sensitivity to H₂O₂ over unmodified electrodes and approached that of well-defined Pt macro disc electrodes. Measured current showed good dependence on concentration, and the calibration curves report good sensitivity of 29.65 nA/cm²/μM for glucose and 8.05 nA/cm²/μM for L-glutamate with a stable, repeatable, and linear response. These findings demonstrate that existing clinical electrode devices can be adapted for combined electrochemical and electrophysiological measurement in patients and obviate the need to develop new electrodes when existing clinically approved devices and the associated knowledge can be reused. This accelerates the time to use and application of in vivo and wearable biosensing for diagnosis, treatment, and personalised medicine.

Carbon microelectrodes with customized shapes for neurotransmitter detection: A review

Carbon is a popular electrode material for neurotransmitter detection due to its good electrochemical properties, high biocompatibility, and inert chemistry. Traditional carbon electrodes, such as carbon fibers, have smooth surfaces and fixed shapes. However, newer studies customize the shape and nanostructure the surface to enhance electrochemistry for different applications. In this review, we show how changing the structure of carbon electrodes with methods such as chemical vapor deposition (CVD), wet-etching, direct laser writing (DLW), and 3D printing leads to different electrochemical properties. The customized shapes include nanotips, complex 3D structures, porous structures, arrays, and flexible sensors with patterns. Nanostructuring enhances sensitivity and selectivity, depending on the carbon nanomaterial used. Carbon nanoparticle modifications enhance electron transfer kinetics and prevent fouling for neurochemicals that are easily polymerized. Porous electrodes trap analyte momentarily on the scale of an electrochemistry experiment, leading to thin layer electrochemical behavior that enhances secondary peaks from chemical reactions. Similar thin layer cell behavior is observed at cavity carbon nanopipette electrodes. Nanotip electrodes facilitate implantation closer to the synapse with reduced tissue damage. Carbon electrode arrays are used to measure from multiple neurotransmitter release sites simultaneously. Custom-shaped carbon electrodes are enabling new applications in neuroscience, such as distinguishing different catecholamines by secondary peaks, detection of vesicular release in single cells, and multi-region measurements in vivo.