Enhanced dopamine detection sensitivity by PEDOT/graphene oxide coating on in vivo carbon fiber electrodes

Abstract

Dopamine (DA) is a monoamine neurotransmitter responsible for regulating a variety of vital life functions. In vivo detection of DA poses a challenge due to the low concentration and high speed of physiological signaling. Fast scan cyclic voltammetry at carbon fiber microelectrodes (CFEs) is an effective method to monitor real-time in vivo DA signaling, however the sensitivity is somewhat limited. Electrodeposition of poly(3,4-ethylene dioxythiophene) (PEDOT)/graphene oxide (GO) onto the CFE surface is shown to increase the sensitivity and lower the limit of detection for DA compared to bare CFEs. Thicker PEDOT/GO coatings demonstrate higher sensitivities for DA, but display the negative drawback of slow adsorption and electron transfer kinetics. The moderate thickness resulting from 25 s electrodeposition of PEDOT/GO produces the optimal electrode, exhibiting an 880% increase in sensitivity, a 50% decrease in limit of detection and minimally altered electrode kinetics. PEDOT/GO coated electrodes rapidly and robustly detect DA, both in solution and in the rat dorsal striatum. This increase in DA sensitivity is likely due to increasing the electrode surface area with a PEDOT/GO coating and improved adsorption of DA's oxidation product (DA-o-quinone). Increasing DA sensitivity without compromising electrode kinetics is expected to significantly improve our understanding of the DA function in vivo.

Keywords: Carbon fiber; Dopamine; FSCV; Graphene oxide; PEDOT.

Figure 1

Scanning electron microscopy images of a bare CFE (a), a 25 s PEDOT/GO coated CFE (b) and a 100 s PEDOT/GO coated CFE (c). All three images display an 18,000X magnification of the electrode surfaces. The scale bars represent 1 μm.

Figure 2

Electrochemical impedance spectroscopy characterization of PEDOT/GO coated CFEs. (a) Average impedance measurements of (n=3 individual electrodes for each deposition duration) over a 1–100,000 Hz sinusoidal current frequency range. (b) The average impedance at 1 kHz sinusoidal current frequency is significantly altered by PEDOT/GO deposition duration (* one-way ANOVA: F(4,14)=234, p<1e⁻⁹).

Figure 3

PEDOT/GO increases the non faradaic current associated with a 400 V/s FSCV scan rate. (a) The average amplitude of the non faradaic current increases with the duration of PEDOT/GO deposition. The grey arrows depict the potential waveform sweep direction. (b) The average (±SEM) total non faradaic charging current amplitude significantly increases with the duration of PEDOT/GO deposition (* two-way repeated measures ANOVA, PEDOT/GO: F(1,27)=13496, p<1E⁻³⁸, deposition time: F(4,27)=618, p<5E⁻²⁶, interaction: F(4,27)=2069, p<5E⁻³³).

Figure 4

PEDOT/GO increases the faradaic current associated with the detection of DA using FSCV. (a) The average faradaic current induced by a 10 μM DA challenge increases with the duration of PEDOT/GO deposition. (b) Sensitivity for DA is determined as the slope of the linear regression fit of the concentration versus current calibration plot. (c) The average CFE sensitivity for DA significantly increases with the duration of PEDOT/GO deposition (§ two-way repeated measures ANOVA, PEDOT/GO: F(1,27)=2587, p<5E⁻²⁸, deposition time: F(4,27)=53.4, p<2E⁻¹², interaction: F(4,27)=151, p<5E⁻¹⁸). Bonferroni post-hoc comparison reveals that sensitivity significantly increases with each increase in deposition time until the increase from 50 to 100 s (* p>0.05).

Figure 5

Both bare and PEDOT/GO CFEs observe the signature DA CV (a) in response to a 10 μM DA challenge, confirming the detection of DA. (b) The i_p,c/i_p,a ratio is significantly altered by the PEDOT/GO coating and duration (* two-way repeated measures ANOVA, PEODT/GO: F(1,27)=3264, p<2E⁻²⁹, duration: F(4,27)=135, p<5E⁻¹⁷, interaction: F(4,27)=18.3, p<5E⁻⁷). (c) ΔE_p is significantly affected by the PEDOT/GO coating and duration (§ two-way repeated measures ANOVA, PEODT/GO: F(1,27)=12.9, p<0.002, duration: F(4,27)=128, p<5E⁻¹⁷, interaction: F(4,27)=134, p<5E⁻¹⁷).

Figure 6

Longer PEDOT/GO deposition durations slow electrode kinetics. (a) The average IT responses normalized to maximum amplitude reveal that the vertical rise and fall time associated with the introduction and removal of a 10 μM DA bolus increases with deposition time. (b) The average non faradaic charging currents normalized to maximum amplitude also reveal a lag in electron transfer kinetics correlating to PEDOT/GO deposition duration. The extent to which the electrode kinetics are altered is assessed by r² correlation coefficient calculation comparing responses pre- and post PEDOT/GO deposition. PEDOT/GO significantly alters the electrode kinetics of the IT plot (c) at 100 s and of the CV plot (d) at 50 and 100s, as measured by the average r² correlation coefficients (* one-way ANOVA Bonferroni post hoc, p<0.05).

Figure 7

PEDOT/GO increases the current of in vivo DA detection in the rat dorsal striatum elicited by a 3 s, 60Hz MFB stimulation train. The blue trace is the average evoked DA overflow response of n=3 individual bare CFEs collected from three separate animals. The red trace is the average response from n=3 individual 25 s deposition duration PEDOT/GO electrodes collected from an addition three separate animals. The period of MFB stimulation is denoted by the black bar.