Recent advances in gold electrode fabrication for low-resource setting biosensing

Abstract

Gold electrodes are some of the most prevalent electrochemical biosensor substrate materials because they are readily functionalized with thiolated biomolecules. Yet, conventional methods to fabricate gold electrodes are costly and require onerous equipment, precluding them from implementation in low-resource settings (LRS). Recently, a number of alternative gold electrode fabrication methods have been developed to simplify and lower the cost of manufacturing. These methods include screen and inkjet printing as well as physical fabrication with common materials such as wire or gold leaf. All electrodes generated with these methods have successfully been functionalized with thiolated molecules, demonstrating their suitability for use in biosensors. Here, we detail recent advances in the fabrication, characterization and functionalization of these next-generation gold electrodes, with an emphasis on comparisons between cost and complexity with traditional cleanroom fabrication. We highlight gold leaf electrodes for their potential in LRS. This class of electrodes is anticipated to be broadly applicable beyond LRS due to their numerous inherent advantages.

Fig. 1. Typical subtractive photolithography methods involve laborious steps, specialized equipment, and expensive, dangerous chemicals. a. Gold is deposited on a substrate via vacuum deposition. b. The gold-coated surface is covered in photoresist. c. Photolithography is performed through a mask to selectively cure the photoresist. d. The photoresist is developed to selectively remove cured or uncured photoresist. e. The exposed gold is etched away. f. The photoresist is removed, and the substrate now contains a patterned gold coating. Figure reproduced and adapted from ref. (Sui et al., 2020) with permission. Copyright Journal of the Electrochemical Society 2020.

Fig. 2. Screen printed electrodes are made by screen printing conductive ink onto a substrate. (a) A mask is first used to screen print the reference and contact electrodes. (b) A separate mask is used to screen print the gold working and counter electrodes. (c) The final device is assembled with a hydrophobic barrier covering the contact electrodes. Figure reproduced and modified from ref. (Ozkan et al., 2015) with permission. Copyright Springer Berlin Heidelberg 2015.

Fig. 3. Inkjet printed electrode fabrication. (a) Gold nanoparticles (AuNPs) are inkjet printed onto a substrate. (b) The AuNPs are sintered to form a continuous surface. (c) Excess AuNPs are washed away. Figure adapted from ref. (Ko et al., 2007).

Fig. 4. Nanoporous gold electrode fabrication with gold alloy leaf. (a) Gold alloy leaf and silver leaf is adhered to a substrate. (b) A protective layer is inkjet printed on the gold leaf. (c) Unprotected leaf is etched away. (d) Protective layer is washed with acetone to reveal the leaf electrodes that will be dealloyed in nitric acid. Figure adapted from ref. (Hondred et al., 2020) with permission. Copyright Royal Society of Chemistry 2020.

Fig. 5. Pure gold leaf electrode fabrication does not require specialized equipment. (a) Gold leaf adhesive is applied to a side of Fellowes. (b) Gold leaf is applied to the adhesive and cut. (c) The gold leaf stickers are peeled off and placed on a transparency film. Conductive silver paint is applied to the leads of all three electrodes. Figure reproduced and adapted from ref. (Zamani et al., 2020) with permission. Copyright American Chemical Society 2021.