Carbon materials have several attractive characteristics as microelectrodes for electrochemical applications, such as wide potential window, good electrochemical activity, chemical stability, and ease in surface functionalization [1]. The most common carbon microfabrication techniques (i.e. screen printing) produce two-dimensional (2D) electrodes, which limit the detection sensitivity. Hence several 3D microfabrication techniques have been explored in recent years amongst which the carbon MEMS (C-MEMS) technique is the most promising one. C-MEMS is a simple and cost-effective method for carbon electrode fabrication, where a patterned polymer template treated at high temperature (~900°C) in inert atmosphere (N2 or Ar) is transformed into pyrolysed carbon [2]. This process enables fabrication of 2D and 3D electrodes with possibility for tailoring ad-hoc designs and unique sensitivities for specific applications. Due to this, pyrolysed carbon is becoming increasingly attractive for numerous applications, such as novel sensors and scaffolds for cell analysis [3]. However fabrication of a conducting 3D microstructure with feature sizes in the micron-range still remains a challenge.
In this work an optimized UV photolithography and pyrolysis process for SU-8 based on highly controlled exposure dose and modified baking time is presented to obtain multi-layered 3D carbon microelectrodes for electrochemistry (figure 1.A). SU-8 2005 (5.6µm) is spin coated on a Si/SiO2 wafer, soft baked (SB) at 50ᵒC for 30min followed by UV exposure (E1 – 210mJ cm-2) and post exposure bake (PEB) at 50ᵒC for 1h (figure 1.A.a). A second thick layer of SU-8 2075 (76µm) is spin coated, SB at 50ᵒC for 6h and UV exposed (E2 – 149mJ cm-2) (figure 1.A.b). A second partial exposure E3 – 28mJ cm-2 is performed to obtain a suspended layer followed by a PEB at 50ᵒC for 8h (figure 1.A.c). The partial exposure dose at wavelength 365nm and the low temperature (50ᵒC) baking steps plays a critical role in fabricating suspended layer. Non cross-linked SU-8 is developed in propylene glycol monomethyl ether acetate (PGMEA) for 30min (figure 1.A.d). The obtained SU-8 polymer templates are then pyrolysed at 900ᵒC for 1h in an N2 environment to obtain suspended 3D pyrolysed carbon microelectrodes (figure.1.A.e). By sequentially repeating the steps shown in figure 1.A. b, c and d followed by a final development step, a multi-layered polymer template can be obtained which can be pyrolysed to produce 3D carbon microelectrodes (figure 1.A.f). The height of the singe carbon layer (figure 1.A.e) is 21.10µm which includes a suspended layer of 2.25µm. The diameter of circles in the suspended layer is 11.50µm. The small carbon microstructures are shown in figure 1.B with structures of 4µm (mesh) and 18µm (mesh boundaries).
Depending on the desired application, different 3D carbon microelectrodes can be fabricated (figure 1.B). Figure 1.C shows the electrochemical characterization of a three electrodes system, comparing planar and 3D carbon working electrodes. The cyclic voltammograms performed in 10mM ferri-ferrocyanide show higher peak current (2 folds higher) for the 3D microelectrodes compared to the 2D ones (figure 1.C). The 3D microelectrodes potentially increase the overall sensitivity in amperometric monitoring of cell response due to the increase surface area and enhanced interaction with cells [3].
Reference:
[1] R. L. McCreery, “Advanced carbon electrode materials for molecular electrochemistry,” Chem. Rev., vol. 108, no. 7, pp. 2646–2687, 2008.
[2] R. Martinez-Duarte, “SU-8 Photolithography as a Toolbox for Carbon MEMS,” Micromachines, vol. 5, no. 3, pp. 766–782, 2014.
[3] L. Amato, A. Heiskanen, C. Caviglia, F. Shah, K. Zór, M. Skolimowski, M. Madou, L. Gammelgaard, R. Hansen, E. G. Seiz, M. Ramos, T. R. Moreno, A. Martínez-Serrano, S. S. Keller, and J. Emnéus, “Pyrolysed 3D-Carbon Scaffolds Induce Spontaneous Differentiation of Human Neural Stem Cells and Facilitate Real-Time Dopamine Detection,” Adv. Funct. Mater., vol. 24, no. 44, pp. 7042–7052, 2014.