Nanopipet-Based Liquid–Liquid Interface Probes for the Electrochemical Detection of Acetylcholine, Tryptamine, and Serotonin via Ionic Transfer

Abstract

A nanoscale interface between two immiscible electrolyte solutions (ITIES) provides a unique analytical platform for the detection of ionic species of biological interest such as neurotransmitters and neuromodulators, especially those that are otherwise difficult to detect directly on a carbon electrode without electrode modification. We report the detection of acetylcholine, serotonin, and tryptamine on nanopipet electrode probes with sizes ranging from a radius of ≈7 to 35 nm. The transfer of these analytes across a 1,2-dichloroethane/water interface was studied by cyclic voltammetry and amperometry. Well-defined sigmoidal voltammograms were observed on the nanopipet electrodes within the potential window of artificial seawater for acetylcholine and tryptamine. The half wave transfer potential, E1/2, of acetylcholine, tryptamine, and serotonin were found to be −0.11, −0.25, and −0.47 V vs E1/2,TEA (term is defined later in experimental), respectively. The detection was linear in the range of 0.25–6 mM for acetylcholine and of 0.5–10 mM for tryptamine in artificial seawater. Transfer of serotonin was linear in the range of 0.15–8 mM in LiCl solution. The limit of detection for serotonin in LiCl on a radius ≈21 nm nanopipet electrode was 77 μM, for acetylcholine on a radius ≈7 nm nanopipet electrode was 205 μM, and for tryptamine on a radius ≈19 nm nanopipet electrode was 86 μM. Nanopipet-supported ITIES probes have great potential to be used in nanometer spatial resolution measurements for the detection of neurotransmitters.

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

“…21,29,30 Briefly, the diffusion limited steady state current at nanoITIES pipet electrodes can be expressed as 35 $$i=4\text{italicxnFDca}$$where i is the steady-state current, x is a function of the quantity RG = r g / a ( r g and a are outer and inner tip radii, respectively), 35 n is the number of transferred charges, F is Faraday’s constant, a is the radius of the electrode, D is the diffusion coefficient of the neurotransmitter, and c is the concentration of the neurotransmitter. A proposed disk geometry for the nanopipet tip was used for the calculation.…”

“…The current values from three replicate background CVs at the diffusion limited potential of GABA transfer were used to calculate the standard deviation of the background, as done in our previous study. 29,30 …”

A Nanopipette Platform for Delivering Nanoliter Volumes of Acetylcholine

“…After that, the pipettes were back-filled with a filling solution of 5 mM TDDATFAB + 1 mM DB 18 C 6 dissolved in DCE. The filling solution was chosen based on what was available/already prepared at the time of the experiments, however, a solution with just 5 mM TDDATFAB and without DB 18 C 6 would also work for ACh detection as shown in our recent work 39 as ionophore is not needed for ACh detection. A platinum wire was inserted into the pipette and fixed with wax.…”

Electroanalytical Opportunities Derived from Ion Transfer at Interfaces between Immiscible Electrolyte Solutions

“…The cationic form of tryptophan transferring across NB−water interface was also studied using an acidic aqueous solution. Colombo et al [98] reported the detection of acetylcholine, serotonin and tryptamine on the nano-ITIES formed at nanopipettes with sizes ranging from a radius of ca. 7 nm to 35 nm, using CV and amperometry.…”