Carbon fiber-microelectrodes (CFMEs) have been the standard for neurotransmitter detection for over forty years. However, in recent years, there have been many advances of utilizing alternative nanomaterials for neurotransmitter detection with fast scan cyclic voltammetry (FSCV). Recently, carbon nanotube (CNT) yarns have been developed as the working electrode materials for neurotransmitter sensing capabilities with fast scan cyclic voltammetry. Carbon nanotubes are ideal for neurotransmitter detection because they have higher aspect ratios enabling monoamine adsorption and lower limits of detection, faster electron transfer kinetics, and a resistance to surface fouling. Several methods to modify CFMEs with CNTs have resulted in increases in sensitivity, but have also increased noise and led to irreproducible results. In this study, we utilize commercially available CNT-yarns to make microelectrodes as enhanced neurotransmitter sensors for neurotransmitters such as serotonin. CNT-yarn microelectrodes have significantly higher sensitivities (peak oxidative currents of the cyclic voltammograms) than CFMEs and faster electron transfer kinetics as measured by peak separation (ΔEP) values. Moreover, both serotonin and dopamine are adsorption controlled to the surface of the electrode as measured by scan rate and concentration experiments. CNT yarn microelectrodes also resisted surface fouling of serotonin onto the surface of the electrode over thirty minutes and had a wave application frequency independent response to sensitivity at the surface of the electrode.
Carbon fiber-microelectrodes (CFMEs) are one of the standards for the detection of neurotransmitters such as dopamine (DA). In this study, we demonstrate that CFMEs electrodeposited with poly (3,4-ethylenedioxythiophene) (PEDOT) in the presence of Nafion exhibit enhanced sensitivity for DA detection. Scanning electron microscopy (SEM) revealed the smooth outer surface morphologies of polymer coatings, which filled in the ridges and grooves of the bare unmodified carbon electrode and energy-dispersive X-ray spectroscopy (EDX) confirmed PEDOT:Nafion incorporation. PEDOT:Nafion coated CMFEs exhibited a statistically enhanced two-fold increase in DA sensitivity compared to unmodified microelectrodes, with stability and integrity of the coated microelectrodes maintained for at least 4 h. A scan rate test revealed a linear relationship with peak DA oxidative current (5 μ M), indicating adsorption control of DA to the surface of the PEDOT:Nafion electrode. As proof of principle, PEDOT:Nafion coated electrodes were used to detect potassium chloride (KCl)-induced DA release in zebrafish ( Danio rerio ) retinal tissue ex vivo, thus illustrating their applicability as biosensors.
Carbon fiber-microelectrodes (CFMEs) are considered to be one of the standard electrodes for neurotransmitter detection such as dopamine (DA). DA is physiologically important for many pharmacological and behavioral states, but is readily metabolized on a fast, subsecond timescale. Recently, DA metabolites such as 3-methoxytyramine (3-MT) and 3,4-dihydroxyphenylacetaldehyde (DOPAL) were found to be involved in physiological functions, such as movement control and progressive neuro degeneration. However, there is no current assay to detect and differentiate them from DA. In this study, we demonstrate the co-detection of similarly structured neurochemicals such as DA, 3-MT, and DOPAL. We accomplished this through electrodepositing CFMEs with polyethyleneimine (PEI) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymers. This endowed the bare unmodified CFMEs with surface charge, physical, and chemical differences, which resulted in the improved sensitivity and selectivity of neurotransmitter detection. The differentiation and detection of 3-MT, DOPAL, and DA will potentially help further understand the important physiological roles that these dopaminergic metabolites play in vivo .
DNA and RNA have been measured with many techniques but often with relatively long analysis times. In this study, we utilize fast-scan cyclic voltammetry (FSCV) for the subsecond codetection of adenine, guanine, and cytosine, first as free nucleosides, and then within custom synthesized oligos, plasmid DNA, and RNA from the nematode Caenorhabditis elegans . Previous studies have shown the detection of adenosine and guanosine with FSCV with high spatiotemporal resolution, while we have extended the assay to include cytidine and adenine, guanine, and cytosine in RNA and single- and double-stranded DNA (ssDNA and dSDNA). We find that FSCV testing has a higher sensitivity and yields higher peak oxidative currents when detecting shorter oligonucleotides and ssDNA samples at equivalent nucleobase concentrations. This is consistent with an electrostatic repulsion from negatively charged oxide groups on the surface of the carbon fiber microelectrode (CFME), the negative holding potential, and the negatively charged phosphate backbone. Moreover, as opposed to dsDNA, ssDNA nucleobases are not hydrogen-bonded to one another and thus are free to adsorb onto the surface of the carbon electrode. We also demonstrate that the simultaneous determination of nucleobases is not masked even in biologically complex serum samples. This is the first report demonstrating that FSCV, when used with CFMEs, is able to codetect nucleobases when polymerized into DNA or RNA and could potentially pave the way for future uses in clinical, diagnostic, or research applications.
Fast scan cyclic voltammetry (FSCV) used in conjunction with carbon fiber-microelectrodes (CFMEs) is an electrochemical technique used to detect neurotransmitters with a sub-section temporal resolution. This allows for a sensitive and selective subsecond analysis of changes in the brain. Typically, this technique is used to detect catecholamines in the brain, but recently, it has branched out into purinergic signaling and peptide neurotransmitters. This technique is not only able to detect neurochemical changes at a quick timescale, but it is able to detect a wide range of molecules with a high spatiotemporal resolution. This results in having chemical selectivity that may not be available in other electrochemical techniques. The nature of CFMEs also allows for the selective targeting of specific brain regions to ensure that precise measurements can be made. The peptide oxytocin is important in understanding stress, depression and anxiety, and it is one of the main components in clarifying signals sent throughout the brain. Oxytocin plays vital roles for women during childbirth as well as for lactation. Oxytocin is typically analyzed by taking a blood sample and running assays that can take a while to obtain the results. Peptides, like oxytocin, can also be detected through methods like mass spectrometry and various forms of chromatography but are sometimes very time consuming and can be destructive. On the other hand, it has been known that there are some amino acids are redox active and can be measured with electrochemical techniques. The detection of tyrosine-containing peptides using electrochemical techniques, such as oxytocin, can be quite difficult. The short amount of time that these molecules are in the extracellular space and its tendency to foul electrodes when oxidized make it hard to collect accurate and consistent data. Also, there is a higher concentration of more electroactive molecules that would make it harder to be specific for these molecules. However, due to the nature of FSCV, it is possible to change the waveform to be able to prevent the fouling of tyrosine on the surface of the electrode and improve the detection of tyrosine-containing peptides in biological samples. Recently, a modified sawhorse waveform (MSW) has been formulated that is able to specifically detect tyrosine in the methionine-enkephalin (M-ENK) neuropeptide using two specific scan rates in the anodic sweep and a short holding period at the switching potential. The MSW waveform was applied at 10 Hz with the holding potential at -0.2 V, then increased to +0.6 V at 100 V/s, and further increased to +1.2 V at 400 V/s. This potential was held in place for 3 ms before sweeping back down to the folding potential of -0.2 V at 100 V/s. This results in high specificity toward tyrosine and eliminates the fouling on the electrode. In this study, we target the tyrosine residue on oxytocin with a MSW waveform and characterize it with carbon fiber microelectrodes and fast scan cyclic voltammetry. We show the first electrochemical detection of oxytocin with FSCV with high sensitivity and spatiotemporal resolution that is distinguished from tyrosine and other amino acids. This work could potentially help further understand oxytocin’s role in vivo and importance in behavioral and disease states.
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