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. Author manuscript; available in PMC: 2024 Apr 1.
Published in final edited form as: Curr Opin Electrochem. 2023 Feb 9;38:101228. doi: 10.1016/j.coelec.2023.101228

3D printing for customized carbon electrodes

Yuanyu Chang 1, Qun Cao 1, B Jill Venton 1
PMCID: PMC9997447  NIHMSID: NIHMS1872781  PMID: 36911532

Abstract

Traditional carbon electrodes are made of glassy carbon or carbon fibers and have limited shapes. 3D printing offers many advantages for manufacturing carbon electrodes, such as complete customization of the shape and the ability to fabricate devices and electrodes simultaneously. Additive manufacturing is the most common 3D printing method, where carbon materials are added to the material to make it conductive, and treatments applied to enhance electrochemical activity. A newer form of 3D printing is 2-photon lithography, where electrodes are printed in photoresist via laser lithography and then annealed to carbon by pyrolysis. Applications of 3D printed carbon electrodes include nanoelectrode measurements of neurotransmitters, arrays of biosensors, and integrated electrodes in microfluidic devices.

Graphical Abstract

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1. Introduction

In recent years, electrochemistry has joined the 3D printing revolution, with new methods to design electrodes including additive manufacturing and lithography.1 Traditional carbon electrodes were typically made from glassy carbon or carbon fibers, with cylindrical or disk geometries.2 3D printing offers the opportunity to make electrodes in a completely different manner: by designing them from the bottom up. Thus, the shape is customizable, because the designs are drawn in a computer-assisted drawing program rather than based on the physical shape of the material.3

3D printing started with additive manufacturing of plastics, and adding carbon materials makes the material conductive.4,5 A newer form of 3D printing that allows smaller electrodes to be printed is 2-photon lithography.6,7 Electrodes are printed in photoresist via laser lithography, where only the small voxel where the lasers meet is polymerized, and the electrode is annealed by pyrolysis to make a carbon electrode. This review focuses on examples of additive manufacturing and 2-photon lithography manufacturing processes, outlining the limitations and advantages of 3D printing carbon electrodes. Finally, we highlight some of the advantages of 3D printing in making devices and custom electrodes and discuss applications of 3D printed carbon electrodes.

2. Methods for 3D printing Carbon Electrodes

2.1. Additive Manufacturing

The most common method of making 3D-printed carbon electrodes is additive manufacturing, where a mixture of conductive polymer and carbon/graphene is extruded. An early example of this work was 3D printing high-impact polystyrene with carbon nanofibers or graphite to determine lead ions by differential pulse voltammetry (Figure 1A).8 Polylactic acid (PLA) is now the preferred conductive polymer as it has a low printing temperature and is more biocompatible. Pumera’s group pioneered PLA/graphene electrodes as detectors for picric acid or ascorbic acid (Figure 1B).9 Kolivoška’s group used fused deposition modeling (FDM) 3D printing to print polylactic acid with carbon black (CB) electrodes (Figure 1C).10 Munoz’s group used a 3D printing pen instead of a desktop printer to print PLA/CB electrodes with similar properties and showed a good performance for 2,4,6-trinitrotoluene (TNT) sensing via square-wave voltammetry (Figure 1D).11 However, the 3D printing pens are limited in precision.12

Figure 1.

Figure 1.

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Examples of additive manufacturing. (A) Directly 3D printed polystyrene/CNF/graphite composite electrode for Pb2+ sensing in aqueous solution (i-iv on the right represents different ion concentrations).8 (B) PLA/Graphene for ascorbic acid and picric acid detection.9 (C) PLA/Carbon black electrodes fabricated by FDM. The CV is Ru(acac)3 detection.10 (D) 3D printing pen printed PLA/CB electrode for TNT measurement via square-wave voltammetry.11 Panels reprinted by permission from: Panel A Wiley, Panel B American Chemical Society, Panels C, D: Elsevier.

One issue with 3D-printed carbon electrodes is that the electrochemical reactions can be slow, and thus treatments are applied to increase kinetics. Applying a high positive voltage for anodic activation increases the oxygen functional groups2 and defect sites, which enhance the detection of neurotransmitters such as dopamine and serotonin.13 Chemical treatments, such as NaOH, dimethylformamide (DMF), or acetone, also improve the conductivity and kinetics by cleaning the PLA and etching the carbon surface.14 Moreover, combining electrochemical and chemical treatment enhances the signal even more;15,16 for example, applying 2.5 V to 3D printed graphene electrodes in DMF significantly enhances the electrochemical performance compared to DMF only.17 Coating metal particles on the printed electrodes also improves the performance,18 such as gold nanoparticles that enhance the conductivity.19 Biological pretreatment with proteinase K enhances the 3D printed PLA/graphene electrode response for ferricyanide.20 In addition, reagentless treatments also improve electrochemical performance,21,22 such as thermal activation to significantly enhance the electron transfer rate.23 Overall, pretreatment of the 3D-printed carbon electrodes to enhance the performance is essential for electrochemical sensing.

2.2. 2-Photon Lithography

Two-photon lithography is a new method to make 3D-printed electrodes. A substrate is covered with photoresist, which is polymerized by lasers. Only specific voxels polymerize where the two lasers produce photons in the same place; thus the spatial resolution is on the order of 1 μm.24 When the unpolymerized photoresist is washed away, the remaining structure is not electroactive, but it can be annealed to form pyrolyzed glassy carbon by heating to high temperatures.7,25 The pyrolysis procedure also shrinks the structure 3-5 fold. Thus, the electrode features are as small as 500 nm, which is smaller than the resolution of the printer.

The Venton group pioneered making freestanding 2-photon lithography printed electrodes for neurochemical applications.7,25 Figure 2A shows the process flow for the fabrication of 3D-printed carbon electrodes by direct laser writing. The electrodes were printed on small metal wires; the printed structures were 180 μm spheres or cones which shrunk to about 60 μm when they were pyrolyzed. Surface features, such as spikes, could be printed with a diameter of 1 μm or less. To make nanoelectrodes, the niobium wire substrate was etched to 1 μm or less and smaller, tapered tips were printed that were submicron after annealing.25 The Keller group fabricated pyrolytic carbon microelectrodes with improved conductivity by selective direct laser writing (Figure 2B).26 The Li group fabricated strain sensors with high sensitivity and stability by direct laser writing of graphene (Figure 2C).27

Figure 2.

Figure 2

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Electrode fabrication by direct laser writing. (A) Left (a-f): Process flow for the fabrication of 3D-printed carbon electrodes. Right: SEM of the printed carbon microelectrodes.7 (B) Left (a-g) Overview of pyrolytic carbon microelectrodes fabrication by selective direct laser writing. Right: The logo of the Technical University of Denmark (DTU) was written using 20 mW power and 0.3 mm/s scan speed.26 (C) Left: Steps of direct laser writing of graphene. Right: Zoom in SEM figures of the printed sensor.27 Reprinted by permission from, Panel A: Wiley, Panel B MDPI, Panel C: American Chemical Society.

Two-photon lithography offers potential upsides of customized shapes, but there are design limitations. One issue is that tapered tips tend to curl during annealing. Figure 3A gives examples of structural deformation for high aspect ratio geometry. The structure is stable with an apex angle about 20 degrees,25 but the tip curls with higher aspect ratios (Figure 3A, b-c). To avoid curling, the nanolattice and printing density can be adjusted,28-31 enabling a feature size of pyrolytic carbon down to 100 nm.32,33 To fabricate high aspect ratio structures, the structure can be mounted with spatial restrictions-such as making a needle tip connected to a base that would limit deformation.34

Figure 3.

Figure 3.

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Limitations for 2-photon lithography. (A) Curling may occur at a high aspect ratio structure during pyrolysis. (a)-(c) are example SEMs and designs of 3D printed tips. (B-E) Illustration for the vertical and horizontal stage setup options: (B) vertical setup and (C) horizontal setup. (D) A proposed strategy for horizontal setup to connect the structure at the beginning of the printing process, and (E) illustration of shadow effect.

Another issue is that of shadows and attachment to the wire. Shapes generally need to be printed from the bottom up, as there is a shadow effect and the lasers cannot print under an object. Figure 3B illustrates a vertical setup, where the metal wires are bent vertically to the stage and the printing axis (z-axis) is the same as the alignment of the wire, so the printed structure attaches to the metal wire from the beginning and there is no shadow. However, it is very challenging to bend the metal wires to a perfectly vertical position, and any deviation from the vertical direction results in asymmetrical printing. In the future, a mold to assist the vertical mounting of metal wires would be helpful. An easier fabrication method is a horizontal set-up of the stage (Fig. 3C). The wires are laid on the wafer with the tip hanging off the end, allowing batch fabrication without specialized mounting. However, the printing process starts on the underside of the wire but the structure does not contact the metal wire until printing is half done (Figure 3C, bottom panel). When the structure is not attached to the wire, it floats in the liquid photoresist and can move out of its intended position. Figure 3D describes a possible solution: printing a small cylinder attached to the wire first as an anchor, and then printing the desired structure upward. Figure 3E shows how the shadow is created under the wire, since the laser power cannot pass through the metal wire; smaller tips have less shadow effect.

3. Advantages and applications of 3D printing

3.1. Integrated Fabrication of Electrodes and Devices.

One advantage of 3D printing is that the electrodes can be printed into devices as they are made. 3D printers with dual extrusion properties embed conductive carbon electrodes into nonconductive polymeric devices, with rapid prototyping and customization on demand. For example, the Kokkinos group designed a single-step fabrication of an integrated 3-electrode 3D printed device (Figure 4A).35 The Bergamini group utilized 3D-printed electrodes as a new platform for immunoassays (Figure 4B).36 The Martin group fabricated a 3D-printed microfluidic chip with directly embedded electrodes (Figure 4C).37 The Rajaraman group combined 3D-printed microelectrode arrays and 3D-printed nerve-on-a-chip as a biocompatible device for cell culture in vitro (Figure 4D).38 The Coltro group also fabricated an 8-electrode device used for adrenaline analysis.39 While integration is easiest with additive manufacturing, silicon probes with multiple functionalities could be printed with 2-photon lithography. Thus, one of the main advantages of 3D printing is that it facilitates the integration of electrodes into devices.

Figure 4.

Figure 4.

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3D printed devices and sensors for applications. (A) A 3D printed electrochemical cell that contains a 3-electrode system for different analyte measurements.35 (B) 3D printed electrodes as a platform for Immunosensor buildup to detect a virus.36 (C) Embedded dual working electrode arrays with microfluidic devices fabricated by additive manufacturing.37 (D) 3D microelectrode arrays for nerve-on-a-chip device.38 (E) 3D printed wavy electrodes by PDMS-CNT composite ink with good stretchability.40 (F) Multifunctional helical liquid sensor by solvent-cast 3D printing.41 (G) Multi-material 3D printed piezoelectric nanocomposite lattices with different geometry designs for pressure sensing.42 (H)SEM images of direct laser writing for flexible piezoresistive sensor arrays fabrication.43 Scale bar for the top right is 500 μm, the top left is 20 μm, the bottom left is 100 μm, and the bottom right is 5 μm. Panels A, B, G and H were reprinted by permission from Elsevier, Panels C and F were from Royal Society of Chemistry, Panel D from American Chemical Society. Panels E was reprinted by permission from Wiley.

3.2. Customized and flexible electrode design

Traditional carbon-based electrodes are fabricated as disk or cylindrical shapes but 3D printing allows customization of the electrode shape. For example, the Yan group utilized 3D printing for a tunable and stretchable CNT electrode with excellent resistance stability (Figure 5A).40 The Therriault group fabricated a multifunctional helical liquid sensor by solvent-cast 3D printing. The multi-walled CNT composite enhanced the electrical conductivity and the helical shape traps the solution for sensitive liquid sensing (Figure 5B).41 Moreover, a flexible 3D piezoelectric nanocomposite containing polyurethane and carbon black was also fabricated by 3D printing for pressure sensing (Figure 5C).42 The Liu group utilized direct laser writing to create porous graphitic structures and fabricated flexible piezoresistive sensor arrays (Figure 5D).43 A new method called stereolithography (SLA) with pyrolysis uses a laser beam to cure a printed photopolymer resin layer-by-layer, constructing a 3D shape.44 This method was used to make freestanding carbon electrodes with complex designs, like a cubic mesh. The Venton lab fabricated freestanding carbon-based microelectrodes by 2-photon lithography 3D printing for neurochemical applications, with cone, sphere, and nanospike modified designs.7 Moreover, carbon-based nanodisk electrodes were also fabricated via 3D printing combined with other treatments and applied for dopamine detection in the Drosophila brain.25

3.3. Batch manufacturing of more complex electrodes

One of the advantages of 3D printing carbon electrodes is that they can be batch fabricated reproducibly. Ironically, it is also easy to prototype, as designs are relatively easy to produce in software and then can be printed quickly. Once a design is finalized, the printer will reproducibly print that design hundreds or thousands of times. The field is not yet at the manufacturing scale, but initial papers showed the printing design with 2-photon lithography showed a 1% variance when printing a sphere.7 However, the shape was not the exact size of the design, so differences in design and actual printing need to be considered before batch manufacturing.

Future applications of customized sensors

Customized electrodes combined with different devices have been broadly applied in multiple fields for electrochemical detection, from neurochemistry to metal sensing to immunosensing. For example, a tunable and stretchable carbon electrode would be a good sensor for gut measurement as 3D printing technology improves.46 A freestanding electrode with nanoscale resolution will also enable direct monitoring of synaptic neurotransmitter release in the future. 3D printed carbon microelectrode arrays (MEA) can be applied for multiple site detection in tissues or cell culture with good biocompatibility. Microfluidic devices with 3D-printed carbon MEAs or biosensors provide the opportunity for rapid, multistep analysis on one device. Finally, 3D printed electrodes provide an opportunity for fundamental studies by customizing the electrode, for example, to study thin layer cell behavior.3 Thus, 3D-printed electrodes have the potential to revolutionize numerous biological and environmental monitoring fields to enable new and better electrochemical analysis.

Acknowledgement

The research from the Venton lab was funded by NIH R01NS125663

Footnotes

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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