Nanoporous gold peel-and-stick biosensors created with etching inkjet maskless lithography for electrochemical pesticide monitoring with microfluidics†
Nanoporous gold leaf (NPGL) is an attractive material for flexible electrochemical biosensors due its high and tunable surface area, electrical conductivity, biocompatibility/non-reactive nature, and rich surface chemistry. However, NPGL synthesis and patterning protocols are complex, costly and consequently not suited for flexible electronic fabrication. This work develops a new manufacturing technique coined Etching Inkjet Maskless Lithography (E-IML) to synthesize and pattern NPGL and metal leaf materials (thickness ∼100 nm) for flexible electronics. E-IML utilizes the versatility of an inkjet printer to pattern electrodes for rapid prototyping, even on flexible substrates (polyimide). We demonstrate how NPGL electrodes, with feature and pores sizes down to approximately 25 nm and 5 nm respectively, can be synthesized and patterned from gold/silver leaf material with E-IML and dealloyed (silver removed) via electrochemical etching. Additionally, a pseudo-reference electrode was E-IML patterned from silver leaf and chlorinated with a diluted bleach solution to make a Ag/AgCl electrode for use in 3-electrode electrochemical biosensing with NPGL working and counter electrodes. These 3-electrode electrochemical biosensors were patterned on adhesive polyimide films for use as disposable peel-and-stick tape biosensors or wearable sticker biosensors. In order to demonstrate the utility of the peel-and-stick biosensors, a disposable tape pesticide biosensor and reusable 3D printed flow cell were developed for organophosphate detection in soil samples. Multiple NPGL working electrodes were fabricated on single devices so that that each electrode could be functionalized with distinct concentrations of the enzyme acetylcholinesterase. Paraoxon (a model organophosphate) sensing results demonstrated a low detection limit (0.53 pM) and high sensitivity (376 nA nM−1). The unique multi-electrode enzyme functionalization protocol allowed for a wider paraoxon sensing range (four orders of magnitude: 1 nM–10 μM) than one electrode alone. The flow cell platform biosensor was also tested in real-world samples (soil slurry) and demonstrated a signal recovery of approximately 93.5% and 91.5% for soil slurry samples spiked with 10 nM and 1 μM paraoxon concentration respectively. Hence these thin-film E-IML NPGL patterning and synthesis techniques could be useful for a wide variety of applications including electrochemical sensors, supercapacitors, batteries, fuel cells, energy harvesters, triboelectric nanogenerators, and membranes.