Removal of Differential Capacitive Interferences in Fast-Scan Cyclic Voltammetry

Abstract

Due to its high spatiotemporal resolution, fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes enables the localized in vivo monitoring of subsecond fluctuations in electroactive neurotransmitter concentrations. In practice, resolution of the analytical signal relies on digital background subtraction for removal of the large current due to charging of the electrical double layer as well as surface faradaic reactions. However, fluctuations in this background current often occur with changes in the electrode state or ionic environment, leading to nonspecific contributions to the FSCV data that confound data analysis. Here, we both explore the origin of such shifts seen with local changes in cations and develop a model to account for their shape. Further, we describe a convolution-based method for removal of the differential capacitive contributions to the FSCV current. The method relies on the use of a small-amplitude pulse made prior to the FSCV sweep that probes the impedance of the system. To predict the nonfaradaic current response to the voltammetric sweep, the step current response is differentiated to provide an estimate of the system’s impulse response function and is used to convolute the applied waveform. The generated prediction is then subtracted from the observed current to the voltammetric sweep, removing artifacts associated with electrode impedance changes. The technique is demonstrated to remove select contributions from capacitive characteristics changes of the electrode both in vitro (i.e., in flow-injection analysis) and in vivo (i.e., during a spreading depression event in an anesthetized rat).

An implantable multimodal sensor for oxygen, neurotransmitters, and electrophysiology during spreading depolarization in the deep brain

“…35 However, the massive change in ionic concentrations during SD and the adsorption of ions to the electrode alters the double-layer capacitance, thus changing the background current. 27, 36 The altered capacitance manifests most strongly as peaks at the switching potentials of the background-subtracted CVs because the polarity of the scan reverses, necessitating the discharge and recharge of the capacitance. The capacitive peaks are then time-delayed because of the low-pass filtering of the UEI.…”

“…27 The negative holding potential (−0.4 V) pre-concentrates dopamine at the electrode surface, and the extended positive limit (+1.3 V) prevents fouling of the electrode surface and enhances adsorption of dopamine. 44 The +0.1 V potential step probes the impedance changes occurring at the time of each waveform application, and convolution of the triangular wave with the derivative of this response predicts the corresponding background signal (or generated charging current) in the existing impedance state.…”

“…27 For this method, the waveform was also scanned at 400 V/s. The scans were preceded by a 1.5-ms step from −0.4 to −0.3 V and back to −0.4 V for 1.5 ms.…”

“…An in-house LabVIEW (National Instruments, Austin, TX) program uses the electrode’s current response to the +0.1 V step to predict and remove interfering impedance effects from the measured CVs, revealing the faradaic signal. 27 …”

Design Choices for Next-Generation Neurotechnology Can Impact Motion Artifact in Electrophysiological and Fast-Scan Cyclic Voltammetry Measurements

“…Motion of the electrode/electrolyte interface during FSCV disturbs the established electric double layer and thus may lead to changes in the capacitive charging current as well as the distribution of charged species, such as dopamine near the FSCV electrode surface [ 68 , 69 ]. In addition, movement of the electrode could change the equilibrium concentrations of dopamine and dopamine-o-quinone (oxidation product of dopamine) near the electrode that is formed as a result of oxidation/reduction, mass transfer, and adsorption dynamics near the electrode [ 68 , 70 ]. Upon motion, the electrode might be leaving the equilibrium formed by those dynamics and entering the bulk concentration that has a higher ratio of dopamine to dopamine-o-quinone, thus resulting in a measured transient increase that decays back to baseline as equilibrium re-establishes.…”