Selective Passivation of Three-Dimensional Carbon Microelectrodes by Polydopamine Electrodeposition and Local Laser Ablation

Abstract

In this article, a novel approach for selective passivation of three-dimensional pyrolytic carbon microelectrodes via a facile electrochemical polymerization of a non-conductive polymer (polydopamine, PDA) onto the surface of carbon electrodes, followed by a selective laser ablation is elaborated. The 3D carbon electrodes consisting of 284 micropillars on a circular 2D carbon base layer were fabricated by pyrolysis of lithographically patterned negative photoresist SU-8. As a second step, dopamine was electropolymerized onto the electrode by cyclic voltammetry (CV) to provide an insulating layer at its surface. The CV parameters, such as the scan rate and the number of cycles, were investigated and optimized to achieve a reliable and uniform non-conductive coating on the surface of the 3D pyrolytic carbon electrode. Finally, the polydopamine was selectively removed only from the tips of the pillars, by using localized laser ablation. The selectively passivated electrodes were characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy methods. Due to the surface being composed of highly biocompatible materials, such as pyrolytic carbon and polydopamine, these 3D electrodes are particularly suited for biological application, such as electrochemical monitoring of cells or retinal implants, where highly localized electrical stimulation of nerve cells is beneficial.

Keywords: 3D pyrolytic carbon microelectrodes; electrodeposition; laser ablation; photolithography; polydopamine; pyrolysis.

Figure 1

Schematic of carbon microelectrode fabrication: (A) SU-8 2035 is spin coated and soft baked; (B) 1st UV exposure and post exposure bake; (C) second layer SU-8 2075 is spin coated and soft baked; (D) 2nd UV exposure and post-exposure bake; (E) Development in PGMEA; (F) Pyrolysis under nitrogen at 1100 °C for 5 h; (G) e-beam deposition of Pt using a shadow mask; (H) SU-8 2005 passivation layer is spin coated and patterned by UV photolithograpy; (I) Top view of the microelectrode chip: (a) after SU-8 pyrolysis, (b) after Pt deposition and (c) after SU-8 passivation layer; (J) Electropolymerization of dopamine by CV; (K) Selective removal of polydopamine by laser ablation.

Figure 2

(a) Cyclic voltammograms for 2D (dashed lines) and 3D electrode at 50 mVs⁻¹ (in blue) and 100 mV s⁻¹ (in pink) in 10 mM [Fe(CN)₆]^3−/4− solution prepared in 10 mM PBS (pH 7.4); (b) Peak currents versus square root of the scan rate.

Figure 3

Monitoring cyclic voltammograms during the electropolymerization at: (a) 50 mV s⁻¹; (b) 20 mV s⁻¹; (c) 10 mV s⁻¹; (d) Plot of the second anodic peak current of the CV of the electropolymerization of dopamine at different scan rates as a function of the number of cycles.

Figure 4

(a–c) SEM images of the bare 3D electrode; (d–f) SEM images of the 3D electrode coated with polydopamine at 10 mV s⁻¹ for 50 cycles.

Figure 5

Cyclic voltammograms of the 3D pyrolytic carbon electrodes after electropolymerization of PDA for 50 consecutive potentiodynamic cycles at different scan rates of 50, 20 and 10 mV s⁻¹. The CVs were obtained in 10 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution prepared in 10 mM PBS (pH 7.4) at 50 mV s⁻¹ scan rate.

Figure 6

SEM images with different magnifications and viewing angles of the micropillars coated with PDA after laser ablation and selective removal of the PDA from their tips.

Figure 7

(a) Cyclic voltammograms of the bare, PDA-coated and laser-ablated electrodes in PBS solution (10 mM, pH 7.4) containing 10 mM [Fe(CN)₆]^3−/4−redox probe at 50 mV s⁻¹; (b) EIS spectra for the bare, the PDA-coated and laser-ablated electrode and Randles equivalent circuit.